ASPECT-A PROBLEM FOR MT 
by BARBARA GAWRONSKA 
Dept. of Linguistics, 
University of Lund, Sweden 
E-mail: linglund@ gemini.ldc.lu.se 
FAX:46-(0)46 104210 
Introduction 
Russian and Polish, two of the five languages 
involved in the experimental MT-system SWE- 
TRA (Dept. of Linguistics, Lund University; 
cf. Sigurd & Gawroriska-Werngren 1988) are 
known as typical aspect languages. The lexical 
inventory of both Russian and Polish contains 
aspectually marked verb pairs, i.e. each verb 
(except a small group of biaspectual verbs) is 
inherently either perfective or imperfective. The 
distinction is usually marked by a prefix (Pol. 
czyta(/przeczyta(, R. ~itat'/pro?itat" 'to read' 
imp/perf) or a change in the stem (Pol. 
podpisa(/podpisywa(, R. podpisat'/podpisyvat' 
'to sign' perf/imp, Pol. bra(/wziqF, R. brat'/ 
vzjat' 'to take' imp/perf). This means that a 
translator formulating a Polish/Russian equiva- 
lent of an English VP almost always has to 
choose between two members of a certain verb 
pair. Human translators who are native 
speakers of Russian or Polish normally perform 
this task without difficulties. What cues are they 
using when deciding which aspectual variant 
fits into the given context properly? Can the 
principles for aspect choice be formalized and 
used in an MT-system? 
The aspect category as a 
linguistic problem 
Do all languages express the category of aspect 
in some way? What exactly is expressed by this 
category? Questions like these have been dis- 
cussed in an enormous number of works in 
general linguistics. Nevertheless, little agree- 
ment has been reached as to the status and the 
meaning of the aspect category. Some of the 
most common controversies in the domain of 
aspectology may be summarized as follows: 
1) Shall aspect be treated as a universal cat- 
egory or as a language-specific one? 
2) Is aspect a purely verbal category, a 
sentence operator, or primarily a dis- 
course strategy? 
3) Is it possible to ascribe an invariant 
meaning to a certain aspect value? Or 
must the meaning of an aspectually 
marked verb be derived from the se- 
mantic features of the verbal stem? 
Each of the questions above has been answered 
in different ways. Several aspectologists are fo- 
cusing on the discourse functions of aspect 
(Hopper & Thompson 1980, Wallace 1982, 
Paprott6 1988); others concentrate on aspect 
choice in isolated sentences (e.g. DeLancey 
1982). There are arguments for an invariant dif- 
ference between the perfective and the imper- 
fective aspect (Forsyth 1970) as well as for in- 
vestigating verbal stems one by one in order to 
discover the meaning of the aspect category 
(Apresjan 1980). 
Despite all controversies concerning the 
status and the main function of aspect, most re- 
searchers agree with the opinion that the perfec- 
tive aspect is normally chosen when referring to 
events, processes or states (the general term 
"event-situations" will be used from now on), 
which are limited, complete or countable, 
whereas the imperfective aspect alludes to un- 
completed event-situations without clear tempo- 
ral boundaries. This way of describing the dis- 
tinction between the perfective and the imper- 
fective aspect is to be found both in traditional 
descriptive grammars (the Soviet Academic 
Grammar 1954) and in recent papers of cogni- 
tive grammarians (e.g. Langacker 1982, Pa- 
prott6 1988). The later authors argue especially 
for a parallelism between mass names and im- 
perfective verbs and between countable nouns 
and perfective verbs. The basic conceptual dis- 
tinction between spatially limited (countable) re- 
ferential objects and referents without clear spa- 
tial limits (denoted by mass names) is assumed 
to apply 'also to the temporal limits of event-ref- 
erents: temporally bounded events become 
"countable", i.e. perfective, and get the "figure" 
(fore.ground) status in a discourse, while event- 
sltuauons which lack temporal limits ("mass" 
referents) are expressed by imperfective verbs 
and function as discourse background. 
The view on the aspect category (at least in 
Polish and Russian) presented in this paper is 
partially related to the interpretation proposed 
AcrEs DE COLING-92, NANTES, 23-28 ao~r 1992 6 5 2 PROC. ON COLING-92, NANTES, AUG. 2.3-28, 1992 
by cognitive grammarians. A similarity between 
typical NP-referents and "event-referents" is 
also assumed, but instead of treating the perfec- 
tive/imperfective distinction as reflecting the 
conceptual difference between "count" and 
"mass" referents, I prefer to relate the aspect va- 
lue to another referential feature, namely, to the 
notion of uniqueness. 
The "uniqueness-based" approach 
The PROLEK~ implementation of some rules for 
aspect choice in translation from Swedish or 
English into Polish/Russian is based on the as- 
sumption that the choice between the peffective 
and the imperfective aspect in Russian and Pol- 
ish reflects the distinction between event-situa- 
tions which are marked as highly specific, 
unique, and those which are unmarked as to 
their uniqueness. By "unique" I roughly mean 
"not identical with another referent in the cur- 
rent universe of discourse from the sender's 
point of view". In the Germanic languages, the 
referents of noun phrases may be marked as 
unique by the definite article or other definite- 
ness markers, e.g. possessive and 
demonstrative pronouns. The uniqueness 
marking may apply both to countable and un- 
countable referents: the dog is sick refers to a 
specific entity belonging to the species dog; the 
wine was good alludes to a specific appearance 
of the substance in question (e.g. the wine that 
has been drunk at a specific party). In Russian 
and Polish, a similar function is fulfilled by the 
perfective aspect-with the difference that the 
choice of a perfective verb marks the referent of 
the whole predication (an event-situation) as 
highly specific, unique, i.e. not identical with 
other event-situations named in the discourse. 
The distinction between the uniqueness hy- 
pothesis and the mass/count interpretation of 
aspect proposed by cognitive grammarians may 
seem very subtle. Nevertheless, it is of import- 
ance. The mass/count analogy does not account 
for some "untypical" cases of aspect use, which 
poses difficulties to adult learners of Russian or 
Polish, e.g. the use of the imperfective aspect in 
Russian/Polish equivalents to a sentence like 
Have you already had breakfast/lunch~dinner? 
(R. Ty u\[e zavtrakal/ obedal/u~inal?, Pol. 
Jadte~ ju~ ~niadanie/obiad/kolacj~?). The event 
referred to is undoubtedly finished and time-li- 
mited, i.e. countable, yet in spite of these 
features, it is expressed imperfectively. The use 
of the perfective variants of the verbs 
exemplified is more restricted: it is e.g. possible 
in situations where the sender stresses the 
importance of the fact that a very specific food 
portion has, so to speak, disappeared, or when 
a sequence of specific events is expressed, as in 
the example below: 
R. My poobedali, 
we ate-lunch-perf 
a potom pogli v kino 
and later went-perf to cinema 
'We had eaten lunch and then we went to the 
cinema' 
Here, the perfective aspect points out that the 
lunch referred to was a unique one (it was fol- 
lowed by the action of going to the cinema), 
whereas in questions like: 
R. Ty u~e obedal? 
you "already ate-dinner-imp 
the sender is not interested in a unique case of 
eating dinner, but merely in whether the ad- 
dressee is hungry or not; thus, the imperfective 
aspect is a natural choice, although the event 
alluded to is a countable one. 
Finding uniqueness cues 
The role of the notion of uniqueness can be 
further illustrated by a fragment of an English 
text translated into Russian by a human transla- 
tor. To make the example clearer, I do not quote 
the whole Russian text, but only specify the 
aspect values chosen by the translator. 
Sample text 
(the initial sentences of the preface to "An 
Introduction to Descriptive Linguistics" by 
H.A. Gleason; aspect values from a translation 
into Russian): 
1.1 Language is one of the most important 
and characteristic forms of human be- 
haviour. 
(no aspect marking - a verbless predicative) 
1.2 It has, accordingly, always had a place 
in the academic world. (imperf) 
1.3 In recent years, however, its position 
has changed greatly. (perf) 
The sample text shows that there is no clear cor- 
relation between the English tense and the Rus- 
sian aspect: the aspect value may vary, although 
the tense value of the source text is constant (in 
both 1.2 and 1.3 the Present Perfect is used). 
Thus, tense cannot be used as a primary cue 
when generating aspect. But if we look for 
uniqueness indices in the source text and treat 
them as aspect indices, the result will be quite 
AC1T~S DE COL1NG-92, NANT~, 23-28 AO~q" 1992 6 5 3 PROC. OF COLING-92. NANTES. AUG. 23-28. 1992 
adequate. In sentence 1.2 (It has, accordingly, 
always had a place in the academic world), the 
adverb always indicates that the predication 
does not refer to any unique situation-the state 
expressed by 1.2 may be true at any point in 
time. Hence, the imperfective aspect is the only 
possible alternative (Polish and Russian perfec- 
tive verbs in the past tense normally do not co- 
occur with adverbs such as always, often etc.). 
The situation expressed in 1.3 (In recent years, 
however, its position has changed greatly) con- 
talns several elements that make it contrast with 
the one named in 1.2. The effect of contrast is 
achieved by the adverb however and by the se- 
mantics of the finite verb changed. In addition, 
the state referred to in 1.3 is placed in a quite 
definite time period (in recent years). All these 
factors taken together provide a sufficient moti- 
vation for marking the referent of 1.3, in the 
given context, as an event-situation which is 
unique in relation to the generally true state 
mentioned in 1.2. Accordingly, the perfective 
aspect is used. 
The sample text shows that there are certain 
adverbials which, on their own, may be suffi- 
cient as aspect indices (as always) and that the 
appropriate aspect value may be indicated by an 
interplay between adverbial phrases, semantic 
features of the main verb, and the context of the 
current predication (1.3). 
An attempt to formalize some 
principles for aspect choice 
A computer program for aspect choice in trans- 
lation should take into account at least those 
types of aspect indices that have been observed 
in the sample text discussed above. The result 
will obviously not be a full set of aspect gene- 
rating rules. Nevertheless, an attempt to design 
an automatic procedure generating aspect is of 
practical and theoretical interest: the translation 
quality may be improved, and an analysis of the 
advantages and the shortcomings of the proce- 
dure may provide a deeper insight into the na- 
ture of the aspect phenomenon. 
The program presented here is implemented 
in LPA MacProlog and functions as an inter- 
mediate (transfer) stage in the translation pro- 
cess-it intervenes between the parsing of the 
Swedish or English text and the generation of 
its Russian or Polish equivalents (similar to the 
procedure for definiteness choice, outlined in 
Gawro6ska 1990). For different language 
pairs, slightly different variants of the transfer 
program are used, but all modules are based on 
the same main principle. 
The programs used for parsing and genera- 
tion are written in a modified version of Refer- 
ent Grammar (Sigurd 1987), called Predicate 
Driven Referent Grammar (PDRG). The forma- 
lism, implemented in DCG, is an eclectic one: it 
is reminiscent of GPSG (no transformations, 
use of LP-rules in parsing certain constituents, 
a GPSG-inspired treatment of relative clauses), 
LFG (the use of c-representations and f-repre- 
sentations) and HPSG (the head of the phrase, 
especially the finite verb, plays the central role 
in the selection of the other phrasal elements). It 
is just the treatment of the finite verb (or a verb- 
less predicative) as the central element of a sen- 
tence that the name of the formalism alludes to. 
A PDGRG rule may be written as follows: 
rsent(d, 
c_rep(\[advp(A),Cat 1 (RI ,Markl), 
vfin(Verb,Aspec0, 
Cat2(R2,Mark2)\]), 
f_rep(\[Frolel(F_Reprl), 
pred(Verb), 
F role2(F_Repr2), 
advl(Af)\]), 
sem_rep( \[event nucl(Event), 
S_role 1 (S Repr 1), 
S_role2(S Repr2), 
circumstances(Feat(As))\])) 
ropadvp(A,Af, As,Feat), 
rconst(Cat 1 (R 1 ,F Repr 1, S_Repr 1 ,Mark 1 )), 
vfin(Form,Aspect), 
{rlex(Form,Verb,v,fin,Aspect ....... 
f_roles(\[F_role 1 (Cat 1,Markl), 
pred(Verb), F role2(Cat2,Mark2)\]), 
s roles(\[S role 1 (F_role 1 ), 
S_role2(F_role2), 
event nucl(Event)\]) }, 
rconst(Cat2(Repr2, 
F_Repr2, 
S Repr2, 
Mark2)). 
d = declarative 
rsent = Russian sentence 
ropadvp = Russian optional adverbial phrase 
rvfin = Russian finite verb 
rconst = an obligatory syntactic constituent 
rlex = Russian lexical entry 
ropadvp = Russian optional adverbial phrase 
f_roles = functional roles 
s_roles = semantic roles 
event nucl = the semantic kernel of the 
p~lication 
The rule above is slightly simplified-it contains 
no agreement conditions and only one optional 
adverbial phrase. In the actual program, the 
ACRES DE COLING-92, NANTES, 23-28 AOt~T 1992 6 5 4 PROC. Or COLING-92, NANTES, AUG. 23-28, 1992 
number of adverbials may vary, and the 
subject-verb agreement is controlled. 
As the result of parsing, three kinds of rep- 
resentations are delivered: 
1) a categorial representation (c rep), 
which is the most language-specific 
one. It contains the information about 
the following facts: 
a. the surface word order 
b. the syntactic category of the com- 
plements of the verb 
c. the case value of the NPs, if present 
d. the form and the case demand of 
valency-bound prepositions, if any 
(this kind of information is repre- 
sented by the variables Markl and 
Mark2) 
2) a functional representation (f rep), in- 
cluding such traditional functional 
roles as subject, object, predicate and 
adverbial 
3) a semantic representation (s_rep), con- 
taining semantic roles like actor, pa- 
tient, experiencer, stimulus, etc. 
The rule above is a very general one: both the 
functional and the semantic roles (F rolel/2, 
S_rolel/2) and the information about their sur- 
face realizations (Cat(egory)l/2) are unspeci- 
fied; in the parsing/generation process they are 
instantiated by utilizing the information stored 
in the lexical entry for the verb (the entity with 
the functor "rlex"), which may have the 
following shape: 
rlex(udaril, 
m(hit,past), 
v,fin,perf,_, 
agr(Isg,ma\]),_, 
f_roles(\[subj(np,nom), 
pred(m(hit,past)), 
obj(np,acc)\]), 
s roles(\[actor(subj), 
patient(obj), 
event nucl(m(hit,past))\])). 
The aspect category is represented both in the 
lexical entry and in the verbal slot of the catego- 
rial representation. The Russian/Polish aspect is 
thus treated as a language-specific category 
marked on the verb, as distinguished from the 
more abstract category of uniqueness, which, 
according to our approach, is a universal con- 
ceptual notion, expressed in different ways by 
different language systems. 
In the translation process, the f-representa- 
tion and the s-representation are utilized. After 
parsing an English/Swedish sentence, the pro- 
gram tries to find out the "uniqueness value" of 
the event expressed by the current predication 
using three main kinds of rules: 
1) rules checking uniqueness indices inside 
the functional and the semantic repre- 
sentation without looking at the con- 
text or using knowledge representation 
stored in the data base 
2) rules comparing the current predication 
with the infommtion about the most 
typical predication containing the cur- 
rent verb (i.e. rules using a knowledge 
representation). The most typical pre- 
dication is to be understood as a de- 
~ription of the most typical event-sit- 
uation, which may be expressed by 
means of the current verb and its com- 
plements. In the data base, such de- 
scriptions are stored as entities with 
the functor proto_event. 
3) rules comparing the cmTent predication 
with its context and inferring the prob- 
ability of aspect change. 
The three kinds of rules apply in the order sug- 
gested above. If a rule of type 1) results in in- 
stantiating the uniqueness value of the event- 
referent as "uni(que)" or "not uni(que)", the 
other rule types do not apply. It means that 
rules of type 1) have to discover the strongest 
"not-uniqueness" indices, like indefinite fre- 
quency or durativity adverbials, or other "not- 
uniqueness" indicating markers, like the 
English progressive tenses, "aspectual" verbs 
like begin, stop etc., or, in Swedish, 
constructions with coordinated verbs (as satt 
och liiste, lit. sat and read - 'was reading') 
which are semantically similar to the English 
progressive tenses. 
This kind of rule may be exemplified by the 
following one, which may be used for finding 
habituality markers like indefinite frequency ad- 
verbials, adverbials expressing durativity or the 
verb brukade ('used to') in the Swedish input: 
uniqueness ind(past,sem_rep(Slist),not_uni):- 
in list(Functor(Repr,Feature)),Slist), 
uniqueness_relevant(Fu nctor), 
not unifl'ense,Functor,Feature). 
"Slist" is the semantic representation (formu- 
lated as a Prolog list). The predicate "in list" 
checks if an element is a member of the list 
Slist. The functor of a list member (Functor) 
may be defined (iu the data base) as potentially 
relevant for the uniqueness value (unique- 
AC~'ES DE COLING-92, NANTES, 23-28 AOl~q" 1992 6 5 5 PROC. OF COLING-92, NANTES, AUG. 23-28, 1992 
hess_relevant). For example, functors like "fre- 
quency" or "durativity", or "actionkind" are 
treated as uniqueness-relevant. Thus, if the se- 
mantic representation Slist contains an element 
like: "action_kind(m(use,past), habituality)", 
i.e. the representation of the verb brukade, or 
"frequency (often,indef)", i.e. the representa- 
tion of the adverb ofta ('often'), the program 
must check whether the combination of the 
functor, the feature specified inside the brackets 
(like "indef" or "habituality") and the tense 
value (here: past) results in a specific unique- 
ness value. As the data base contains the fol- 
lowing information: 
not_uni(past,frequency,indef). 
not_uni(_,_,habituality). 
the program will decide that a sentence in the 
past tense containing an adverb like ofta or a 
finite verb like brukade does not refer to a 
unique event-situation. As a consequence, the 
imperfective aspect will be preferred when gen- 
erating the target equivalent. 
The next step, given the semantic represen- 
tation and the uniqueness value, is to create a 
new functional representation, if needed, and 
then the appropriate c-representation. Some- 
times, the input and the output may have the 
same f-representation, and differ only as to 
some details in their c-representations, like e.g. 
simple transitive sentences: 
Sw. pojken slog ofta hunden 
the-boy hit often the-dog 
Pol. chtopiec czgsto bit psa 
boy-nom often hit-imp dog-acc/gen 
R. malZik ~asto bil sobaku 
boy-nom often hit-imp dog-ace 
f rep(\[subj(m(boy,sg)),pred(m(hit,past), 
advl(often,indef)\]) 
But in such cases as the Swedish construction 
with brukade there is a need for changing the 
functional representation, as the most natural 
way of expressing the feature "habituality" in 
the Russian or Polish equivalent is by using the 
imperfective aspect and (optionally, if the habit- 
uality should be emphasized) an adverb like 
usually. Such changes are not especially diffi- 
cult to implement if the semantic representation 
is used as a kind of interlingua. In the s-repre- 
sentation, the infinitive following the habituality 
marking verb brukade is treated as a semantic 
kernel of the event situation. The program must 
therefore find the target equivalent of the 
semantic kernel, make it the main predicate, 
provide the target representation with the right 
aspect value and then, optionally, insert an ad- 
verbial as an extra habituality marker. These 
operations result in translations like: 
Sw. Han brukade komma ffr sent 
he used come to late 
Pol. Zwykle sig spdlniat 
usually refl-he-was-late-imp 
R. On obydno opazdyval 
he usually was-late-imp 
Rules belonging to types 2) and 3) take care of 
cases lacking such obvious uniqueness indices 
as in the example above. Type 2) has access to 
the proto_events, i.e. representations of typical 
predications containing a certain verb. A 
proto_event may have the following structure: 
proto_event (become engaged, 
\[actors(\[specific,limited_reff2)), 
durativity(limited), 
frequency(low,def), 
uniqueness(high)I). 
A type 2) rule applying to a predication contain- 
ing the predicate meaning 'be engaged' checks 
whether the "actors" involved are two specific 
individuals and whether there is no violation of 
the other conditions specified in the description. 
If the current predication matches most of the 
elements specified in the frame "proto_event", 
the uniqueness value of the "proto event" (here: 
uniqueness (high), which means: unique with a 
high degree of probability) will be ascribed to 
the current event-referent. This means that, 
when translating a Swedish meaning like Per 
och Lisaffrlovade sig ('Per and Lisa became 
engaged') the perfective aspect would be cho- 
sen, whereas the same Swedish verb used in a 
sentence like: Fi~rr i tiden ffrlovade folk sig pd 
ffrgildrarnas order ('in former times, people got 
engaged by order of their parents') would be 
rendered by the Russian/Polish imperfective 
verb. 
The following is an example of a type 2) 
rule: 
uniqueness_ind(past,sem_rep(Slist),not_uni):- 
in list(event_nucl(m(EventNucl_)),Slist), 
proto_event (EventNucl ,Condlist), 
in list(uniqueness(high),Condlist), 
not(cond matching(Slist,Condlist)). 
The rule states that if the proto_event containing 
the semantic kernel of the current predication 
(EventNucl(eus)) is specified as unique with a 
high degree of probability and if the relevant 
ACRES DE COLING-92, NANTES. 23-28 Ao~r 1992 6 5 6 PROC. OF COLING-92. NANTES. AUG. 23-28. 1992 
elements of the semantic representation of the 
current sentence do not match the conditions 
stored in the proto_event, then the uniqueness 
value of the event-situation referred to is ."not 
unique". Writing specific rules matching 
semantic representations with proto_events is 
obviously not a trivial task-there are not many 
event-situations which are as easily described as 
the case of being engaged. 
Type 3) rules are the most complicated 
ones, as the task performed is to compare the 
current predication both with the proto event 
and with the previously stored semantic repre- 
sentations (including their uniqueness values) in 
order to discover possible motivation for aspect 
change. For the time being, only a restricted 
number of cues have been implemented. The 
program utilizes principles like: 
--It is quite probable that parts of a unique 
event may also be unique, if no 
counter-indices (as e.g. indefinite du~ 
rativity markers) have been found. 
--A predication which describes the man- 
ner of performing an already intro- 
duced event should probably be 
treated as imperfective (it expresses a 
property of an event-referent, in a way 
similar to a predicative NP: it does not 
introduce a new referent, but ascribes 
a property to an already introduced 
one). 
--Adverbials marking a kind of opposition 
(however etc.) and their interplay with 
other adverbials may be important 
cues for aspect change. 
Conclusions 
The main problems when implementing a pro- 
cedure for aspect generation are to formulate 
concise and coherent descriptions of typical 
events, to design an appropriate hierarchy of 
rules comparing the current predication with the 
proto events and to describe conditions for 
aspect change. This is a field for further re- 
search. Another area for future investigations is 
finding cues for aspect choice in constructions 
containing infinitives where the infinitive is not 
preceded by an aspectual verb like the verbs 
meaning start or finish. Nevertheless, some 
uniqueness indices are possible to formalize and 
to implement in an MT-system (obviously, a 
system accepting lexical and syntactic restric- 
tions). Our approach is a kind of compromise 
between different points of view represented in 
current research on aspect: the overt aspect is 
treated as language-specific, but the conceptual 
distinction behind the aspect choice is assumext 
to be based on the universal notion of unique- 
ness; furthermore, both seutence-internal and 
contextual factors are taken into consideration. 
The compromise seems to be quite useful. 

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