Representing temporal discourse markers for generation purposes 
Brigitte Grote 
Otto-von-Guericke Universit~it Magdeburg 
Institut fiir Wissens- und Sprachverarbeitung 
P.O. Box 4120, 39016 Magdeburg, Germany 
emaih grote~iws, cs. uni-magdeburg, de 
Abstract 
Discourse markers are an important means to signal 
the kind of coherence relation holding between adja- 
cent text spans. Research on generating discourse 
markers has been mainly concerned with causal 
markers, whereas temporal markers have not received 
much attention. In this paper, we identify semantic, 
pragmatic and syntactic features that are required 
to support a motivated choice of German temporal 
subordinating conjunctions and prepositions during 
text production. Information on individual markers 
is assembled in a discourse marker lexicon, which is 
used as a declarative resource at the sentence plan- 
ning stage. We illustrate how this resource can be 
used to produce alternative verbalizations of the tem- 
poral relationship holding between two events. 
1 Motivation 
In text, discourse markers signal the kind of coher- 
ence relation holding between adjacent text spans. 
For any but the most trivial applications of language 
generation, motivated marker choice is an important 
task. Whereas several studies have been concerned 
with causal markers and their interactions with other 
linguistic means, for instance, Vander Linden and 
Martin (1995), Rhsner and Stede (1992), Delin et al. 
(1996), temporal markers as signals of the temporal 
relation holding between two events have not received 
much attention, with the exception being Dorr and 
Gaasterland (1995). However, quite often, it is only 
by means of explicit temporal markers that the cor- 
rect interpretation of a text can be ensured, as the 
following examples illustrate: 
(1) Der Abstand ist nachzumessen, nachdem das Band 
mindestens einen Umlauf ausge~hrt hat. (Check 
the distance after the belt has completed at least 
one round.) 
(2) W~ihrend Sie den Toaster betreiben, die 
Brotschlitze nicht abdecken. (While operating the 
toaster, do not cover the bread slots.) 
In both cases, the order of events as recounted in 
the text does not correspond to their order of oc- 
curence: In example (1), nachdem (after) marks the 
event denoted in the second clause as temporally an- 
terior to the one denoted in the first clause. In ex- 
ample (2), wiihrend (while) marks both events as 
cotemporal. Without a temporal marker, the order 
of events would not be obvious. 
Selecting an appropriate discourse marker for a 
given temporal relation is by no means a straightfor- 
ward matter, though. First, one and the same tem- 
poral relation is verbalized differently depending on 
the syntactic and lexical properties of the clauses it 
conjoins. In German, the language we consider, such 
properties are, among others, tense, aspect, and syn- 
tactic structure. Interdependencies occur when two 
temporal events are realized in the same sentence; we 
will therefore restrict the study of German temporal 
markers to subordinating conjunctions and preposi- 
tions. Second, temporal discourse markers can even 
overwrite the temporal relations indicated by other 
linguistic means like tense and aspect, as noted by re- 
searchers who work in the analysis of temporal mark- 
ers (e.g. Hitzeman et al. 1995). 
In a nutshell, when aiming at selecting an ap- 
propriate temporal marker in text generation one 
needs a representation of temporal markers that en- 
ables marker choice and other sentence level decisions 
(such as tense and aspect selection) to mutually con- 
strain each other. In this paper, we take up the idea 
suggested in Grote and Stede (1998) of a discourse 
marker lexicon as a declarative resource at the sen- 
tence planning stage. We demonstrate how such a 
lexicon for temporal markers can be employed in text 
generation to produce different verbalizations of the 
same underlying temporal organization depending on 
other generation decisions. 
The paper is organized as follows: Section 2 re- 
views related work on generating temporal mark- 
ers. Section 3 describes the major semantic, prag- 
matic and syntactic properties of German temporal 
prepositions and subordinating conjunctions. Sec- 
tion 4 presents the generation perspective: It briefly 
discusses the shape of the discourse marker lexi- 
con, introduces the features used in the lexicon, and 
presents sample lexicon entries and their application 
in the generation process. 
22 
2 Related work 
Work on discourse marker generation in general has 
focussed on marker selection, mainly for causal re- 
lations (Elhadad and McKeown 1990; Vander Lin- 
den and Martin 1995), and on the realization of 
RST's subject-matter relations (Rhsner and Stede 
1992; Delin et al. 1996). As for temporal mark- 
ers, Dorr and Gaasterland (1995) examine the gen- 
eration of English temporal subordinating conjunc- 
tions. Gagnon and Lapalme (1993), on the other 
hand, describe the generation of French temporal ad- 
verbs based on a DRT representation of the discourse. 
While Gagnon and Lapalme (1993) only briefly 
address conjunctions and prepositions, Dorr and 
Gaasterland (1995) present a detailed study of tem- 
poral connectives, but they consider English mark- 
ers only. The only account on automatically produc- 
ing German temporal expressions that we know of is 
Ehrich (1987); however, she discusses the interaction 
of tense and aspect in simple sentences only. 
Most studies that deal with discourse markers re- 
gard their production as a mere consequence of other 
sentence level decisions such as aggregation, lexical- 
ization, syntactic structuring, and--in the case of 
temporal markers--as determined by tense and as- 
pectual choices. We believe, however, that one needs 
a more flexible control to increase the expressiveness 
of generation systems. Although there have been 
quite a few studies on individual aspects of sentence 
planning, little attention has been paid to the in- 
teraction between the various tasks--exceptions are 
Rambow and Korelsky (1992) and Wanner and Hovy 
(1996)--and in particular to the role of marker choice 
in the overall sentence planning process. 
There exists a large body of research in NLU on 
analysing the temporal structure of texts, including 
the role of temporal markers, though again restricted 
to English (Moens and Steedman 1988; Lascarides 
and Oberlander 1993; Hitzeman et al. 1995). We turn 
to these studies when it comes to identifying the in- 
formation that needs to be assembled for representing 
temporal markers. 
3 Linguistic perspective: 
Describing temporal markers 
Selecting an appropriate German temporal marker 
given two events in a temporal relationship requires 
detailed knowledge of the semantic, pragmatic and 
syntactic properties that characterize temporal mark- 
ers. This section introduces the major properties and 
explores the correlations between temporal markers 
and other linguistic means that indicate temporal or- 
ganization. We base our account on two sources: 
descriptive linguistic studies, mainly by Helbig and 
Buscha (1991), B/iuerle (1995), Buscha (1989) and 
Steube (1980); and our analysis of temporal marker 
usage in the German LIMAS corpus (Glas 1975). 
3.1 The 'meaning' of German temporal 
markers 
Temporal subordinating conjunctions and temporal 
prepositions conjoin two events where the event in the 
subordinate clause (or the PP) provides the tempo- 
ral framework for interpreting the event in the main 
clause: Bevor Sie den Toaster reinigen, den Net- 
zstecker ziehen. (Before you clean the toaster, un- 
plug the device.) and the corresponding 'shorthand' 
form l br dem Reinigen des Toasters den Netzstecker 
ziehen (Unplug before cleaning the toaster). 
Semantic properties German grammars such as 
Helbig and Buscha (1991) list about 20 temporal 
subordinating conjunctions and 20 temporal prepo- 
sitions. Their semantics is usually described by the 
kind of temporal relation they establish between two 
events, see for instance, Steube (1980) and Helbig and 
Buscha (1991): The event in the main clause can ei- 
ther overlap with (simultaneity), succeed (anteri- 
ority), or precede (posteriority) the event depicted 
in the subordinate clause or the prepositional phrase. 
In table 1 we provide a synthesis of the classifica- 
tions of the most frequent German temporal mark- 
ers by Helbig and Buscha (1991), Buscha (1989) and 
B/iuerle (1995). The markers listed in the table re- 
flect the scope of the marker study in this paper. 
Two aspects are especially prominent: First, each 
of the three temporal relations can be realized by a 
number of temporal markers. Alternatives within a 
class differ in that they realize some additional mean- 
ing aspect. Consider the markers of simultaneity: 
Solange, for instance, conveys the idea of a strict si- 
multaneity where two events have the same start and 
end time, and is more specific than w~hrend; sooft, 
to give another example, highlights the concurrence 
of two events. 
Second, table 1 shows that some markers are am- 
biguous: Als and wenn occur in all three classes, seit- 
dem, sobald and sooft in two. Apparently, neither 
of them has any special temporal implicature on its 
own; instead, these markers depend on syntactic and 
lexical contexts to receive an umambiguous temporal 
meaning. We will return to this issue in section 3.2. 
Pragmatic properties The choice Of a particular 
marker to express a temporal relation between two 
events interacts with the focus structure as in: 
(3) (a) Bevor ihr Mann das Haus verlie\[J, ging sie zur 
Arbeit. (Before her husband left the house, she 
went to work.) 
(b) Nachdem sie zur Arbeit gegangen war, verliefl 
ihr ivIann das Haus. (After she had gone to work, 
her husband left the house.) 
Alternatives (3a) and (3b) both express that the 
event of 'going to work' precedes the event of 'leav- 
ing the house'. They differ in that they focus on 
23 
temporal relation temporal markers 
simultaneity subc: als (as), indes(sen) (meanwhile), seitdem (since), sobald (as soon as), solange 
(as long as), sooft (whenever), sowie (as soon as), w~hrend (while), wenn (when) 
prep: an (at), auf (on), bei (during), binnen (within), durch (for), in (in), iiber (over), 
w~hrend (during) 
anteriority subc: a./s (when), kaum dab (no sooner), nachdem (after), seit(dem) (since), 
sobald (as soon as), sooft (whenever), sowie (as soon as), wenn (when) 
prep: ab (from), nach (after), seit (since) 
posteriority subc: als (when), bevor (before), his (until), ehe (before), wenn (when) 
prep: his (until), vor (before) 
Table 1: German temporal subordinating conjunctions (subc) and prepositions (prep) classified by temporal 
relations. Note that the corresponding English markers are only approximate translations. 
different aspects of the situation: In (3a) the ear- 
lier event is in the centre of attention, in (3b) the 
later one (assuming that the matrix sentence is more 
prominent). This phenomenon interacts with other 
discourse phenomena, for instance, given and new 
information, and--when placed in a larger discourse 
context--with presuppositions and their accommo- 
dation (Lascarides and Oberlander 1993). However, 
the treatment of the discourse behaviour of temporal 
markers is beyond the scope of this paper. 
Pragmatic issues further concern style. Regard- 
ing temporal markers, stylistic features are of minor 
importance: We only observe variation between ar- 
chaic and neutral (da vs. als), and formal and neutral 
(kaum dab vs. sobald) markers. 
3.2 Syntactic and lexical constraints 
When expressing several events in the same sentence, 
marker choice interacts with other linguistic means: 
Temporal markers impose particular constraints on 
the syntactic and lexical contexts they can occur in. 
Conversely, these contexts can influence the meaning 
of markers. 1 
Markers and Aktionsart/aspect Aspect is tra- 
ditionally taken to have two components, the non- 
inherent grammatical features, and the inherent lex- 
ical features. Inherent features characterize facets of 
the situation denoted by a verb, for instance, whether 
it is an event or a state. We will label these fea- 
tures Aktionsart to avoid confusion. According to 
Bussmann (1990), the major Aktionsarten in German 
are stative (wissen/to know) and dynamic. For the 
latter, the basic dichotomy is that between durative 
(schlafen/to sleep) and non-durative verbs, which are 
subdivided into iterative (flattern/to flap), semelfac- 
tive (klopfen/to knock), resultative (verbrennen/to 
burn up) and causative verbs (trii.aken/to water). 
1Traditional grammars, which the present account is based 
on, usually list aspect, Aktionsart and tense as constraining 
parameters on marker choice. However, there is no consensus 
on the role of these parameters; B~.uerle (1995) provides a good 
overview of the range of positions. 
Two kinds of interdependencies are generally ac- 
knowledged, see Ehrich (1987), Buscha (1989) and 
B~iuerle (1995). First, temporal markers are sensi- 
tive to the Aktionsart of a verb. Consider w~ihrend 
and als which can both express simultaneity: 
(4) (a) Als das Kabel schmolz / riB, war ich nicht im 
Raum. (When the cable melted / tore, I wasn't in 
the room.) 
(b) W~hrend das Kabel schmolz / *riB, war ich 
nicht im Raum. (While the cable melted / *tore, I 
wasn't in the room.) 
Wghrend expects a durative verb in the subordi- 
nate clause, hence it can occur with schmelzen/to 
melt but not with reit3en/to tear. Als, in contrast, 
can be used with durative and resultative verbs, as 
(4a) illustrates. Second, temporal markers may even 
shift the Aktionsart of a verb, for instance from a 
semelfactive reading to an iterative one as in: 
(5) (a) Wenn es an der Tfir klopft, schreit das Baby. 
(When someone knocks at the door, the baby cries.) 
(b) Wiihrend es an der Tfir klopft, schreit das 
Baby. (While someone knocks at the door, the 
baby cries.) 
Grammatical aspect reflects the individual per- 
spective a speaker adopts with respect to an event, 
such as perfective (temporally closed) or imperfec- 
tire. In German, this distinction is grammatically 
realized by choosing a perfective or simple tense, e 
Aktionsart and aspect closely interact, consider ex- 
ample (6) where the anterior reading (6b) is due to 
the use of a perfective tense with a non-durative verb 
in the subordinate clause, which indicates that the 
activity has been concluded: 
(6) (a) Seitdem ich ihn kenne, ist er Nichtraucher. 
(Since I know him, he is a non-smoker.) 
(b) Seitdem seine I~rau gestorben ist, sehe ich ihn 
nur selten. (Since his wife has died, I only rarely 
see him.) 
Sin contrast to English, and especially to slavic languages, 
German has no elaborate aspect system: Distinctions like pro- 
gressive and simple cannot be signalled by morphological fea- 
tures of the verb, but require a separate temporal adverb: He 
is reading vs. Sie 1lest gerade (She reads right now). 
24 
Here, verb properties determine the reading of the 
temporal marker. Our study of temporal marker oc- 
currences in the LIMAS corpus suggests that mark- 
ers belonging to the simultaneity class typically real- 
ize imperfective aspect, whereas temporal connecting 
words that signal anteriority correlate with a perfec- 
tive aspect in the subordinate clause. 
Markers and verbal tense Some markers can 
only be used with particular tenses, for instance, a/s 
in its simultaneous reading cannot occur with present 
tense, whereas wenn as signal of simultaneity corre- 
lates with present and past tense: 
(7) (a) Als er in Dresden war (*ist), suchte (,sucht) er 
seine 1~reundin au£ 
(b) Wenn er in Dresden ist / war, sucht / suchte er 
seine Freundin au£ 
However, tempus sensitivity of temporal markers is 
not a matter of the grammatical tense form (such as 
simple past, present perfect, etc.) but relates to the 
temporal structure of the individual events, and to 
how their temporal structures are related. Assum- 
ing the Reichenbachian threefold distinction between 
Event Time (E), Reference Time (R), and Speaking 
Time (S) (the Basic Tense Structure, BTS, (Reichen- 
bach 1947)), we observe that the constraints imposed 
by a marker on verb tense concern the underlying re- 
lation between E and S of both clauses: Selecting 
either a/s or wenn to express simultaneous events in 
the main clause (era) and in the subordinate clause 
(es) depends on whether the event times precede S 
(E(em),E(es)_S) or concur with S (E(em),E(es),S). 3 
The grammatical tense results from combining the 
BTS of both clauses and their aspectual features. 
Markers and syntactic structure The most 
straightforward correlation is that between syntac- 
tic structure and marker choice: If two events are 
expressed by a hypotactic structure, a subordinating 
conjunction is required. When a deverbal realization 
of an event is possible (e.g. treffen/das Treffen; to 
meet/the meeting), a clause with an adverbial (tem- 
poral) prepositional phrase is realized. 
Markers and temporal quantifiers With some 
markers, the temporal relation denoted by the marker 
can be quantified by a temporal adverb as in kurz 
bevor (shortly before) or einige Stunden nachdem 
(several hours after); others cannot be quantified: 
• einige Stunden sobald (.several hours as soon as). 
4 Generation perspective: 
Representing temporal markers 
A representation of temporal markers suitable for 
generation purposes has to accommodate the follow- 
ing demands: First, it has to describe the semantic 
SThe comma stands for 'is cotemporal', the underscore for 
'precedes'. 
and pragmatic features of markers in a manner that 
supports a motivated choice between markers which 
can realize the same temporal relation. Second, it 
has to account for the constraints temporal mark- 
ers impose on their syntactic and lexical contexts, 
thereby enabling interactions between marker choice 
and other sentence planning decisions where the order 
of decision-making is not fixed. In Grote and Stede 
(1998) we argue that such a flexible control is best 
realized by introducing independent modules for the 
different sentence planning tasks, such as proposed 
by Wanner and Hovy (1996), and that these modules 
should rely on declarative representations as much as 
possible. Therefore, we propose a discourse marker 
lexicon, i.e. an independent lexical resource that as- 
sembles specifically the information associated with 
discourse markers. 
Traditional lexicology and grammars describe lexi- 
cal entries along three features: semantic, pragmatic 
and syntactic dimensions (see section 3). From the 
production perspective, these features are to be clas- 
sifted with respect to when and where they come into 
play in the generation process; this amounts to a pro- 
cedural view on the information coded in the lexicon. 
Following Grote and Stede (1998) we assume three 
categories in the marker lexion: 
• Applicability conditions: The necessary con- 
ditions that need to be present in the input rep- 
resentation for the marker to be a candidate. 
Chiefly, this is the semantic/discourse relation 
to be expressed, and also (if applicable) features 
pertaining to presuppositions and intentions. 
• Combinability conditions: The constraints 
that the marker imposes on its neighbouring lin- 
guistic constituents (the 'syntagmatic' dimen- 
sion). These are syntactic constraints on subcat- 
egorization and semantic type constraints, which 
interact with other realization decisions in sen- 
tence planning. 
• Distinguishing features: If preferential choice 
dimensions, such as style, brevity, etc., are at- 
tended to in the system, then these features serve 
to distinguish markers that are otherwise equiv- 
alent (the 'paradigmatic' dimension). 
In the remainder of this section we describe lexicon 
entries for temporal markers along these lines. 
4.1 Applicability conditions 
Semantic conditions The semantic classes intro- 
duced in section 3.1 (simultaneity, anteriority and 
posteriority) turned out to be too coarse for genera- 
tion purposes. Instead, one needs a more fine-grained 
representation of the semantics of temporal markers 
to support an informed choice among markers within 
the broad classes. 
Allen's temporal interval relationships provide an 
adequate framework (Allen 1984), as already sug- 
25 
gested by Dorr and Gaasterland (1995). Allen in- 
troduces seven basic temporal interval relationships, 
namely equals(=), after(>), during(d), overlaps(o), 
meets(m), starts(s), finishes(f)--and their inverses 
<,di, oi, mi, si,fi--that may exist between two events 
em and es. For instance, overlaps(em,e~) as in (4b) 
implies that there is an intersection between the time 
at which em occurs and the time at which es occurs, 
but that neither event is a subset of the other. 
Each temporal relation corresponds to one or sev- 
eral German temporal markers, for instance, overlaps 
may be expressed by the entire range of simultane- 
ity markers given in table 1, except for solange and 
kaum daB. Conversely, the majority of the tempo- 
ral markers can realize several temporal interval re- 
lations. Take the connective nachdem as in example 
(1), which can have the following meanings, 
after(ern, es) A meets-i(em, es) 
or w~ihrend as in example (4b), 
equals(ern, es) A during(era, es) A starts(era, es) 
A finishes(era, es) A overlaps-i(em, es) 
whereas solange has only one reading: 
equals(era, es ). 
This adequately captures the semantic difference 
between w&hrend and solange. In the lexicon, the ap- 
plicability conditions of a particular temporal marker 
are now described by listing the temporal interval re- 
lations it can realize. 
Pragmatic conditions In section 3.1 we briefly 
discussed pragmatic features of temporal markers. 
For the time being, the lexicon supports the features 
style, with the values neutral, brief, formal, 
archaic, and intention. Its value evaluative in- 
dicates the speaker's (negative) attitude towards the 
kind of temporal relation holding between two events 
(Steube 1980; Buscha 1989). 
4.2 Combinability conditions 
Combinability conditions appear as constraints in the 
lexicon entries of individual markers. In the present 
lexicon, constraints are described using the following 
features: 
Aktionsart The Aktionsart plays a central role 
during the lexicalization of events: Candidate verbs 
are, among others, selected due to their Aktion- 
sart. Aktionsart features are usually stored in the 
lexicon entries of verbs, and are thus available to 
sentence planning. To represent these constraints, 
we turn to Bussmann (1990) for the major Aktion- 
sarten in German (see also section 3.2). 4 At present, 
the lexicon supports a subset of Bussmann's Aktion- 
sarten, namely stative, durative, iterative, 
semelfactive, causative and resultative. 
4There is no generally accepted and well-defined set of Ak- 
tionsart features; we opted for Bussmann (1990) because these 
features are supported by the lexicalization component we in- 
tend to use (Stede 1996). 
Aspect Grammatical aspect is encoded using the 
feature values perfective and imperfective. 
Tense We argued above that marker choice relates 
to the underlying temporal structure---as expressed 
in terms of the Reichenbachian threefold description 
of time-and not to a particular grammaticai tense 
(see also Ehrich (1987)). Temporal constraints in the 
marker lexicon will thus be described using the BTS 
notation, and defining the legal linear orderings of 
E, R and S of the related events. For instance, als 
in its simultaneous meaning imposes the constraint 
E(e~),E(e~)_S, which can be realized by all gram- 
matical tenses that meet this constraint. 
Mapping this representation into grammatical 
tense requires knowledge on how to map pairs of Ba- 
sic Tense Structures to the tense structure of complex 
German sentences, as described in Hornstein (1990) 
for English (Complex Tense Structures, CTS) and ex- 
tended by Dorr and Gaasterland (1995) to cover in- 
tervals, too. Since we envision independent modules 
for the different sentence planning tasks that posit 
their choices as constraints, the tense selection pro- 
cess need not concern us. 
Syntactic structure Possible values are pp 
(prepositional phrase) and subord (subordinate 
clause); both refer to the realization of the event that 
acts as temporal reference point. 
Quantification The lexicon contains the two val- 
ues quantifiable and not-quantifiable. 
4.3 The shape of the lexicon 
The possible values for the applicability and combin- 
ability features can now be used in the lexicon to 
describe individual temporal markers. Table 2 gives 
the lexical representations for most of the German 
anteriority markers and the posteriority marker be- 
fore. Similar representations have been developed 
for the other marker classes given in table \]. si- 
multaneity and posteriority. Feature values for in- 
dividual markers have been identified by analysing 
marker occurrences in the UMAS corpus (Glas 1975); 
as such, they mainly reflect marker usage. We then 
compared our marker descriptions to results from re- 
search literature (see section 3). Note that combin- 
ability conditions can apply to main and subordinate 
clause/prepositional phrase separately, hence some 
feature values are prefixed with me:, sc: and pp: 
to mark their scope. If a marker involves no con- 
straint for a particular feature, the slot in the table 
remains empty. 
Table 2 contains an informal description of the lexi- 
con entries; the formal representation depends on the 
actual sentence planner used in text production, see 
Grote and Stede (1998) for a preliminary proposal. 
26 
Feature nachdem (after) nach (after) sobald (as soon as) kaum dab (no sooner bevor (before) 
applicability 
- denotation after(e,,, e~)A after(e,,, e,)A meets-i(e,~, e~) meets-i(e,~, e, ) before(e~, e~)A 
meets-i(em, e,) meets-i(e~, e~) meets(e~, e,) 
combinability 
- Aktionsazt 
- aspect 
- tense 
sc:resuitativeA 
sc:iterativeA 
sc:semelfactive 
sc:perfective 
mc:imperfective 
E(e,)._E(e,n)_SA 
E(e~)...E(e~), SA 
E(e,)3_E(e,) 
subord 
pp:resultativeA 
pp:iterativeA 
pp:semelfactive 
{E, S}(e~) 
= {E, S}(e,)A 
E(e~)_E(e.)_S 
subord 
E(e,)..E(e~)_SA E(e,), E(e,,)_S 
sc:non-durative 
{E, S}(e~) 
= {E, S}(e,) 
- syntax pp subord subord 
- quantifier quantifiable I quantifiable not-quantifiable not-quantifiable quantifiable 
preferences 
- style neutral brief neutral formal neutral 
- intention evaluative 
Table 2: Lexicon entries for some German temporal markers 
4.4 Selecting temporal markers 
This section briefly addresses the issue of selecting an 
appropriate temporal marker during text production 
using the discourse marker lexicon. We will focus on 
the anteriority markers. 
In our scenario, generation starts from a conceptual 
representation which contains the facts that must be 
reported in the text and their position in time. Let 
us assume the following very simple input structure: 5 
el: arrive (he, home, 19 : 14) 
e2: watch(he ,TV, 19 : 15,22 : 30) 
The first event precedes the second event, but they 
'meet' at one point in time. Now, the first step is 
to determine the applicable temporal relations. Two 
interpretations are possible, depending on the dis- 
course context and focus structure, which we have 
not dealt with so far: Focussing on the earlier event 
would yield the temporal relation meets(era, es), with 
em = el, focussing on the later event the relation 
meets-i(e,n, e,), with e,n = e2. Matching this against 
the lexicon entries in table 2 would produce bevor in 
the former case, and nachdem, nach, sobald, kaum 
dat3 as candidate realizations for the latter interpre- 
tation. Possible verbalizations are: 
(8) (a) Bevor er Fernsehen Euckte, ist er navh Hause gekommen. 
(Before he watched TV, he has come home.) 
(b) Sobald er nach Hause gekommen war, guckte er 
Fernsehen. 
(As soon as he had come home, he watched TV.) 
(c) (Direkt) nachdem er nach Hause gekommen 
war, gucckte er Fernsehen. 
SThis is an abridged representation. We will eventually 
represent the facts as SitSpecs (Stede 1996), which will be an- 
notated with temporal information. During lexicalization--as 
one task in the sentence planning phase---SitSpecs are mapped 
onto semantic representations (SernSpecs). 
((Right) after he had come home, he watched TV) 
(d) Nach dem Heimkommen guckte er Fernsehen. 
(After coming home he watched TV.) 
(e) Kaum dab er nach Hause gekommen war, 
guckte er Fernsehen. 
(As soon as he had come home, he watched TV.) 
Assuming the anteriority interpretation (Sb-e), 
how does a generation system choose among the four 
remaining alternatives? We argued above that we 
envision a modular architecture where independent 
sentence planning modules posit their constraints re- 
garding tense selection, lexicalization, syntactic real- 
ization, etc. 6 In case no constraints are put forward 
by the sentence planning modules, sobald (Sb) would 
be selected, as it is the most specific and at the same 
time neutral realization. If, however, a quantifier is 
to be included, then nachdem would be chosen i~c). 
If brevity is a stylistic concern, and the process in the 
subordinate clause can be deverbalized, a phrasal re- 
alization with the preposition nach is selected (Sd). 
If, on the other hand, a more formal realization is 
the overall goal given to the generator, kaum daft (Se) 
would be chosen. In these cases, marker choice would 
posit constraints (as given in the combinability slot 
in table 2) on all other sentence planning decisions. 
So far, we only considered a perfective aspect in 
the subordinate clause. Once we change aspect to 
imperfective, a realization including nachdem is no 
longer an option, compare ,Nachdem er nach Hause 
kam, hat er Fernsehen geguckt (After he came home, 
he has watched TV). Sobald would be an adequate 
realization. Likewise, changing the Aktionsart from 
resultative to durative, as in Sobald er schlMt, guckt 
6This approach differs from Dorr and Gaasterland (1995) 
who impose a strict order on the selection of tense, aspect and 
connecting word. 
27 
sie Fernsehen (As soon as he sleeps, she watches TV) 
would rule out nachdem. With the resultative variant 
einschlafen (fall asleep) both markers are possible. 
Finally, if a constraint is posited that the tense has 
to be 'present', kaum dab would not be available. 
5 Conclusion and Outlook 
Temporal markers have neither received much atten- 
tion in NLG, nor has a principled account of marker 
selection as such been introduced. In this paper 
we presented a general framework for representing 
German temporal markers for generation purposes. 
We identified some of the features required to de- 
scribe applicability conditions, constraints and pref- 
erences, and proposed a declarative lexical resource 
that makes it possible to treat temporal markers and 
other linguistic means as mutual constraints at the 
sentence planning stage. Now, we need to examine 
individual temporal markers more closely and incor- 
porate the temporal marker lexicon into a text gen- 
eration system. 
For the purpose Of this paper, we have assumed 
that temporal relations are always explicitly sig- 
nalled, and thus limited our study to marker selec- 
tion. Marker occurrence, however, is an important 
issue. First, Hitzeman et al. (1995) argue that there 
exist temporal defaults of the kind "An event will 
occur just after a preceding event"; this renders the 
introduction of explicit markers superfluous. Second, 
we have only assumed pairs of time-stamped expres- 
sions, but have ignored that they usually occur in 
a larger discourse situation where other kinds of co- 
herence relations might hold between events. For in- 
stance, all causal coherence relations have some tem- 
poral implicature; still, one does not want a temporal 
marker to signal a VOLITIONAL-CAUSE, even though 
cause and effect are temporally related. 
Finally, future work needs to address the interac- 
tion of marker choice and temporal adverbs, as these 
are the means to realize the simple/progressive dis- 
tinction in German. 
Acknowledgement Thanks to Manfred Stede and 
two anonymous reviewers for helpful comments on 
earlier versions of this paper. 

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