INTERPRETING SYNTACTICALLY ILL-FORMED SENTENCES 
Leonardo LESMO and Pietro TORASSO 
Dipartimento di Informatica - Universita' di Torino 
Corso Massimo D'Azeglio 42 - 10125 Torino - ITALY 
ABSTRACT 
The paper discusses three different kinds of 
syntactic ill-formedness: ellipsis, conjunctions, 
and actual syntactic errors. It is shown how a new 
grammatical formalism, based on a two-level repr_e 
sentation of the syntactic knowledge is used to cope 
with Ill-formed sentences. The basic control struc 
ture of the parser is briefly sketched; the paper 
shows that it can be applied without any substan 
tial change both to correct and to ill-formed sen 
tences. This is achieved by introducing a mechanism 
for the hypothesization of syntactic structures, 
which is largely independent of the rules defining 
the well-formedness. On the contrary, the second 
level of syntactic knowledge embodies those rules 
and is used to validate the hypotheses emitted by 
the first level. Alternative hypotheses are obtain 
ed, when needed, by means of local reorganizations 
of the parse tree. Sentence fragments are handled 
by the same mechanism, but in this case the second 
level rules are used to detect the absence of one 
(or more) constituents. 
INTRODUCTION 
In the last years we have been involved in 
building a natural language (Italian) interface to 
ward a relational database. Even if this research 
required to consider issues relative to knowledge 
representation (Lesmo et al 83) and query optimiza 
tion (Lesmo et al, in press), our main concern was 
to devise efficient parsing techniques (Lesmo et al 
81, Lesmo & Torasso 83). 
The term "efficient", when applied to language 
processing, can take a number of different meanings, 
ranging from pure processing speed to the ability 
to analyze fragments of text, to the flexibility 
that characterizes the behavior of the parser. We 
believe that all facets of efficiency are worth be 
ing pursued, but if the communication between the 
man and the machine has to occur in a really natu 
ral fashion, the robustness of the parser, i.e. its 
ability to cope with unforeseen inputs must receive 
the greatest attention. It is important to realize 
that "unforeseen" is assumed her to refer to the 
syntactic form of the input sentence: of course, 
also inputs that are unexpected from a semantic 
point of view should be handled properly, but, 
since usually the syntactic knowledge acts as a fil 
ter between the reception of the input and the sub 
sequent stages of the analysis, the first problem 
that must be faced is the following: how can the 
parser be prevented from rejecting sentences that 
are syntactically ill-formed, but could be interpr_e 
ted correctly if they are passed to the other comp2 
nents of the system? 
Alternatively, the problem can be stated as: 
how to foresee every interpretable input? Marcus 
(1982) envisages the following alternatives: 
a) the use of special "un-grammatical" rules, which 
explicitly encode facts about non-standard usage 
b) the use of "meta-rules" to relax the constraints 
imposed by classes of rules of the grammar 
c) allowing flexible interaction between syntax and 
semantics, so that semantics can directly ana 
lyze substrings of syntactic fragments or indi 
vidual words when full syntactic analysis fails. 
Even if we agree in stating the importance of a 
strong interaction between syntax and semantics, 
our approach is quite different from c) (as 
well as from the other ones). For this reason, and 
in spite of the fact that a detailed description of 
the parser's operating principles has been given 
elsewhere (Lesmo & Torasso 83), the next section is 
devoted to an introduction to the basic ideas that 
led to the design of the syntactic knowledge source. 
The subsequent sections will cover some phenomena 
which are related with ill-formedness of sentences, 
namely: ellipsis, conjunctions, and some types of 
actual syntactic errors. 
GRAMMARS AND NATURAL LANGUAGE 
It is widely accepted (see Charniak 81) that 
syntactic knowledge consitutes one of the founda 
tions needed to build natural language interpreters. 
Various kinds of grammatical formalisms have been 
devised to represent in efficient, flexible and pe\[ 
spicuous way the syntactic knowledge (Winograd 83). 
Even if the formalisms are quite different, the 
main characteristic shared by all grammars is that 
they are prescriptive (or normative) in nature. A 
grammar defines what a sentence is, that is it spe~ 
534 
what sequences of words are acceptable. This is in 
sharp contrast with the normal use of language, 
which has, as its main purpose, the communication 
of something. Of course all grammars can be (and 
have been) augmented in order to build a representa 
tion of the meaning of the sentences (i.e. some 
thing that should be able to carry most of its tom 
municative contents), but a meaning can only be ob 
tained for correct sentences. 
Some efforts have recently been devoted to ex 
tending the coverage of grammars, in order to deal 
also with ill-formed sentences (Kwasny & Sondheimer 
81, Weischedel & Sondheimer 82, Granger 82). This 
is usually done by relaxing the constraints imposed 
by some rules of the grammar, by adding new rules 
to take care of some kinds of ill-formedness, or by 
allowing the semantics to intervene when the sy~ 
tax is not able to process the input. However, most 
of these approaches present some problems: either 
the perspicuousness and the readibility of the gram 
mar is reduced or the control structure of the ana 
lyser is made considerably more complex. 
The sources of ill-formedness can be grouped 
in three classes: ellipsis, conjunctions, and syn 
tactic errors. 
In the case of ellipsis, a fragment such as 
"John" or "probably" can be understood by a human 
listener without any particular difficulty, prov! 
dad that a particular context is given. On the oth 
er hand, it is apparent that those fragments are 
not consistent with the rules defining the well- 
formed sentences. 
Similar problems arise in case the grammar at 
tempts to cope with conjunctions. In general, ellip 
sis is meaningful just in case a context external 
to the expression to analyse is assumed to exist. 
The situation with conjunctions is rather different: 
in some sense, the context that must be used to in 
terpret a conjunct is given by the previous con 
junet(s), so that it is expressed inside the sen 
tence that has to be analysed. The difficulty in 
the analysis of conjunctions depends on the fact 
that not only the second conjunct is often ill- 
formed (if it is considered as a standing-alone sen 
tence), but it is the particular form of ill-formed 
hess that provides the analyzer with the piece of 
information needed to decide what is the syntactic 
role of that conjunct (or, if we assume that the re 
sult of the syntactic analysis is represented in 
form of a tree, to decide where the constituent ex 
pressed by the conjunct has to be appended in the 
syntactic tree). For this reason, in the following 
sentences the second conjuncts have quite different 
roles: 
John loves Mary and Susy (i) 
John loves Mary and Susy Fred (2) 
John loves Mary and hates Violet (3) 
Thus, as in the case of ellipsis, a syntactic ana 
lyser designed to handle conjunctions must be able 
to operate on ill-formed fragments, but with the 
additional difficulty of modifying the parse tree 
on the basis of the type of ill-formedness. 
The last source of ill-formedness that we will 
consider are the syntactic errors. Differently 
from the previous cases, it is almost impossible to 
list all possible mistakes that a person could make 
in writing a sentence. Probably, most of them can 
not be considered as syntactic errors (e.g. misspe! 
ling of words or wrong markers for a given case of 
a verb), but there are also errors that have purely 
syntactic grounds. Some noticeable examples are 
agreement errors, ordering errors and errors in 
verb tenses. An examples of each of them is report 
ed below: 
John love Mary (4) 
John is going probably to home (5) 
Yesterday I have eaten a good cake (6) 
Even if a more detailed discussion appears in the 
fifth section of this paper, it is worth noting 
here three points: 
- most native English speakers will probably never 
make such errors, but, firstly, they could easily 
be made by non-native speakers and, secondly, at 
least the error exemplified in (4) could result 
from a typing error 
- errors of that kind are more frequent in Italian, 
since it is richly inflectional 
- even if the first and third type of errors can be 
(more or less) easily handled by means of relaxa 
tion techniques (Kwasny & Sondheimer 81), this is 
not the case for ordering errors; this is due to 
the fact that the agreement and tense constraints 
are expressed "explicitly" in the grammar (e.g. 
by an augmentation), whereas the order is specif_i 
ed implicitly (i.e. rigidly embodied in the gram 
mar itself). 
The analysis of the problems mentioned in this 
section, together with some other considerations 
that are not worth being discussed extensively here 
(regarding, for instance, garden paths) led us to 
the design of a formalism for representing the sy~ 
tactic knowledge that splits it into two levels. 
The first level contains a set of rules that, in 
our intention, characterize the meaningful sen 
fences. It can be questioned whether rules regard 
ing meaning can be considered as syntactic rules. 
Our opinion is that the syntactic categories asso 
ciated with natural language words have a strong 
semantic bias (see, for a thorough discussion of 
this thesis (Lyons 77, Chapt.ll~ For this reason, 
we defined a set of node types that have to be used 
in building the tree representing the syntactic 
structure of the sentence. These node types (report 
ed in table l) are associated with the syntactic 
categories and the topological constraints that go v 
535 
REL Relation Verbs, copulas 
REF Referent Nouns, pronouns 
CONN Connector Prepositions, conjunctions 
DET Determiner 
MOD 
ADJ 
Adverbial 
Modifier 
Adjectival 
Modifier 
Articles 
demonstrative adjectives, 
adjectival question words 
Adverbs 
Adjectives 
Table 1 - The node types: The first column contains 
the name (actual and extended); the sec- 
oond one contains the classical syntactic 
categories associated with the node type 
ern the attachment of nodes constitute the basic 
filter which selects the "meaningful" fragments of 
sentence. As an example of this kind of constraint% 
it is unreasonable to assume that an ADJ node can 
be attached elsewhere than a REF node (with the ex- 
ception of verbs having a copulative function, e.g. 
to be, to seem, to taste etc.). For this reason, in 
dependently of its position in the sentence, we can 
exclude some kinds of constructs (e.g. ADJ-ADJ at- 
tachment) as meaningless. W When a rule of the first 
set is executed it (normally) involves the creation 
of a new node (possibly more than one) and its at- 
tachment to the syntactic tree which was built up 
to that time. 
Because of the limited knowledge used to hypo- 
thesize the attachment point, it can often happen 
that the parser made the wrong choice. Such an er- 
ror can be detected by using two different knowledge 
sources: higher-level syntactic constraints and se- 
mantics. The first of them contains the rules that 
define the well-formedness of sentences (in partic- 
ular gender-number agreements rules and ordering 
rules) whereas the second knowledge source tells 
whether an attachment is semantically acceptable 
(of course, even if a REF-ADJ attachment is consis 
tent with the topological constraints, not all ad- 
jectives can be used to qualify a given noun). The 
semantic checks are done accessing a semantic net 
organized in two levels: the first of them (exter- 
nal) concerns the acceptable surface structures (e. 
g. case frames for verbs), whilst the second one 
(internal) is concerned with the actual semantics 
of the domain (e.g. subsetting among classes). 
4 it must be noted that the rules embodying these 
constraints are expressed in procedural form. Even 
if the lack of a declarative representation makes 
more difficult the design and the maintenance of 
the rules, they are made more efficient in terms 
of execution time by taking into account the con 
text where the word occurs (involving a limited 
one word lookahead). 
Because of the frequency of this kind of wrong hyp2 
thesization, an effective computational tool must 
be used to restructure the tree: this tool consists 
in what we called "natural changes", which are sim- 
ple pattern-action rules able to move around con- 
stituents; their purpose is to provide the parser 
with an alternative hypothesis when a given one has 
failed. Whereas the natural changes are tri~ered 
the same way both in case the inconsistency is syn- 
tactic and semantic, different courses of action 
take place if the changes cannot produce any accep~ 
able alternative hypothesis: if the error is of sy~ 
tactic type than the first hypothesis is maintained 
but a warning message is sent to the user; if the 
error is semantic, then the current interpretation 
of the fragment is considered unacceptable and, in 
case one or more choice points were previously met, 
the parser backtracks, otherwise the analysis fails. 
More details about the use of backup, as well as 
about other topics related with the parsing strate- 
p~y, can be found in (Lesmo & Torasso 83). 
A problem which must be faced when a natural 
change is stimulated is the choice of the best in- 
terpretation. Let us suppose that an agreement be- 
tween an adjective and a noun is violated. In this 
case the natural change MOVE UP tries to attach the 
adjective to a REF node which is at a higher level 
with respect to the REF which the adjective is cur 
rently attached to. The new attachment stimulates 
the rules of the second set (that is the rules veri 
lying the agreement and the word ordering) and the 
semantic ones. It is possible that the semantic 
rules signal that the new attachment is not admissi 
ble from a semantic point of view. At this point, 
if no alternative attachment is possible, the sysL 
tem has to consider the first interpretation as the 
best one since it violates only the "weak" syntac- 
tic constraints. 
ELLIPSIS 
"Ellipsis" is a greek word (elleipsis) roughly 
corresponding to "lack, omission", that is used, to 
take a dictionary definition, to stand for "omis- 
sion of one or more words that can easily be sub- 
sumed". Even if all components of the definition 
are fundamental, we want to stress the presence of 
the adverb "easily". It is consistent with the ob- 
servation that, whereas other phenomena occurring 
in natural language (e.g. garden path) require a 
conscious effort in the listener, elliptical sen- 
tences are understood without any difficulty. On 
the other hand, most current grammatical formalisms 
are not able to account for this ease in understand 
ing ellipsis; it must be noted the importance that 
is often laid on the ability to decide as soon as 
possible what is the allowable form of a given conz 
stituent (Buchenko et al. 83). This is due to the 
necessityof triggering in advance a suitable re- 
536 
stricted set of grammar rules, in our case this is 
not required: the first-level rules will work the 
same way independently of the global context where 
s given word or constituent occurs (this is not 
true for "local" contexts in the current version of 
the system: see note i); the consistency with the 
rules which govern the construction of well-formed 
sentences will be tested afterwards. This is parti- 
cularly useful for handling elliptical fragments. 
Let's see through a pair of examples what is the b~ 
haviour of the parser in such sistuations. 
Example (i) is reported below: 
John (i) 
The rules associated with the category "noun" (note 
that the first-level rules are grouped in packets 
associated with syntactic categories), in case the 
analysis is at the beginning of the sentence, cause 
the building of the sentence reported below: 
REL 
I i,l 
CONN J- 
REF @" 
I JOHN 
When the end of the sentence in encountered, the 
structure is recognized as being incomplete and a 
pattern matching procedure applied to any preceding 
question can reconstruct its actual meaning. What 
must be noticed is that the first-level syntactic 
rules used to analyze the fragment are exactly the 
same that are used to analyze complete and correct 
sentences. 
CONJUNCTIONS 
The kind of processing that occurs in handling 
conjunctions requires the introduction of rather 
different constraints. The first interpretation pro 
duced for sentences 3) and 4) after the fragment 
"John loves Mary and Susy" has been analyzed is re- 
ported in fig. is. This interpretation is confirmed 
when the end of sentence 3) is encountered (so that 
the final structure is the one shown in fig. la). 
On the contrary, when the name "Fred" is scanned in 
sentence 4), it cannot be attached to "Susy" (excl~ 
ding the possibility that "Fred" is her family name) 
and the attempt to move it up to "loves" causes a 
semantic error (three unmarked case for "love"). At 
this point another "natural change" is triggered, 
which handles conjunctions. It tries to move up the 
"and" node, producing the structure of fig.lb which 
is accepted as the correct one. Note, however, that 
this kind of natural change is much more complex 
than the standard ones. For example, in the report- 
ed examples two new nodes have to be built: the emp 
ty REL node (this is done easily since only two 
nodes of the same type can be connected via "and") 
ILOVES h I Hl,l IUN~rl 
I UNMARKED 12 1 
(a) 
(b) 
Fig.l - The parse trees for sentence 3) (fig.la) 
and sentence 4 (fig.lb). 
and the "UNMARKED" connection (for which an explic- 
it request of creation and attachment must be is- 
sued). 
A final observation regards the fact that the 
parser assumes that the first acceptable interpre- 
tation is the right one. This implies that a sen- 
tence of the form (see EX4 in Huang 83, pag.82) 
"The man with the telescope and the woman with the 
umbrella kicked the ball" would be interpreted as 
"The man with the telescope and with the woman with 
the umbrella kicked the ball", that is not the most 
natural interpretation for a human listener. How- 
ever, Italian always expresses explicitly the num- 
ber of the verb (i.e. plural in this case), so that 
the Italian translation of the sentence would be 
analyzed correctly. 
SYNTACTIC ERRORS 
The system tolerates and possibly recovers the 
following different kinds of errors: 
- lexical errors 
- agreement errors 
- errors in the ordering of the constituents 
- extra cases 
(note that only the second and the third kind of 
errors are actual syntactic errors). 
As regards the errors at the lexical level, 
they are detected when the morphological analyzer 
tries to decompose a given word in "root + suffix" 
form. When no decomposition is posslble or none of 
the obtained roots occurs in the dictionary, the 
system asks the user about the possibility that the 
input word is mispelled. In the affirmative case, 
the user can retype the word, whereas in the oppo- 
site case the system asks the user to provide it 
with some pieces of information such as the synta~ 
tic category of the word, its normalized form (i.e. 
its root), the gender, the number, etc.; moreover 
the system asks what semantic object the word re- 
fers to. In this way the analysis of the sentence 
can go on and possibly an interpretation is con- 
structed. However, it has to be pointed out that 
the information provided by the user during the 
537 
analysis of the sentence is not always sufficient 
for the system to complete the analysis. In fact, 
the current version of the system has not the capa- 
bility of restructuring the semantic net dynamical- 
ly, so that the system can continue the analysis 
only when the semantic object denoted by the un- 
known word is already present in the net. 
As regards "agreement errors" there is a large 
variety of error types grouped under this label: 
a) a first kind refers to the agreement in number 
and gender between the noun and the determiner 
and between the noun and the adjectives. It is 
worth noticing that such kind of errors is un- 
common in Italian, because the suffixes for male 
and female and for singular and plural are in 
many cases quite different. 
b) A slightly more frequent error concerns the a- 
greement in number, gender and person between 
the subject and the verb. Since in Italian the 
suffixes indicating the different persons of the 
verb, its tense and mood are quite different, 
people whose mother tongue is Italian usually do 
not make this kind of mistake. 
c) Another kind of agreement refers to the relation 
ships existing between the moods and the tenses 
of the verbs occurring in the main sentences and 
its subordinates. The rules, which are quite com 
plex since they derive from the "consecutio tem- 
porum" of Latin, are often violated so that this 
kind of error must be tolerate by the system. In 
this case the procedure which has the task of 
verifying the agreement emits a warning message 
when the rules are violated, but, contrarily to 
cases a) and b), it does not try to restructure 
the parse tree via "natural changes", since in 
most cases no alternative interpretation exists. 
The framework we have provided is particularly 
useful for treating errors in the ordering of the 
constituents, in fact the order is checked only 
when a given sentence (possibly a subordinate) has 
been completed. This happens when the REL node that 
heads the clause (main or subordinate) is closed, 
that is a punctuation mark is encountered or a new 
node is attached to a node which is (in the parse 
tree) at a level higher than the REL currently ana- 
lized. Before stimulating the ordering rules, the 
system checks that the case frame of REL has been 
correctly filled, that is all the cases attached to 
REL are compatible with the head and among them. 
Just in this case a set of rules is activated de- 
pending on the sentence type (it is apparent that 
the constituent order is different in a declarative, 
interrogative or relative clause). Each rule repre- 
sents a legitimate ordering of the constituents and 
the rules are ordered in decreasing degree of ac- 
ceptability. The rules are matched in turn against 
the actual case frame of the verb acting as head of 
the clause under examination; in case no rule 
matches, a warning is issued to signal the user 
that something has gone wrong in the ordering; any- 
way the interpretation of the clause obtained by ac 
cessing the semantic net is maintained and the ana- 
lysis goes on if the entire sentence has not yet 
been scanned. A similar (but simpler) processing oc 
curs for a REF node with respect to the adjectives 
attached to it. 
There are also cases which are more difficult 
to treat thao the ones involving violations in the 
word ordering. In fact, a sentence like "Ii giorna- 
le Io ha comprato Giovanni stamattina" (literally 
"The newspaper it has bought John this morning") in 
volves not only word order violations (the syntac- 
tic object occurs in the first position in the sen- 
tence), but also there is a case denoted by "io" 
("it") which duplicates the object. Such sentences 
are clearly incorrect from a syntactic point of 
view as well as, in principle, from a semantic one 
(wrong case frame), but they are perfectly under- 
standable and quite frequent because they allow one 
to identify as focus of the utterance the object 
without passivizing the sentence. 
The treatment of such kinds of errors requires 
only relatively inexpensive modifications to the 
way the semantic net is accessed. It is worth no- 
ticing, in fact, that the syntactic object ("il 
giornale") is attached to a REL node which is empty 
when this attachment is performed. The semantic and 
agreement check procedures are stimulated but are 
immediately suspended since the REL node is empty. 
Similarly the pronoun "lo" is attached to the REL 
and the corresponding check procedures are suspend- 
ed. When the REL node has been filled with "compra- 
to" the suspended checks are resumed. The semantic 
procedure is able, by inspecting the semantic net, 
to state that "giornale" may fill the "object" role 
so that when the previously suspended semantic 
check is executed, it concludes that "lo" ("it") 
cannot be attached to the REL filled with "comprare" 
("buy") since the object role has already been fil- 
led. 
Instead of rejecting the current interpreta- 
tion by stimulating the natural changes and possi- 
bly the backup mechanism, a modification of the par 
sing strategy consists in attaching a warning to 
the REF node containing the pronoun "lo" and in go- 
ing on with the sentence analysis. When the sen- 
tence has been completely scanned and, consequently, 
it is possible to perform a global check on the ac- 
tual case frame of "comprare", the semantic proce- 
dure decides that "lo" is simply a repetition of 
the object and therefore it may be disregarded. In 
this way the interpretation of the sentence is pos- 
sible, but the warning attached to the REF node con 
taining "io" is output to the user. 
538 
CONCLUSIONS 
The paper presents a parsing strategy able to 
cope with different kinds of syntactic ill-formed 
hess: ellipsis, conjunctions, syntactic errors. Some 
examples are reported to show that the adopted for 
malism allows the parser to analyse ill-formed fra~ 
ments without substantial changes to the rules used 
to analyse correct sentences. 
However, some problems still deserve further 
attention. First of all, in case of ill-formed sen 
tences it is often possible to assign more than one 
interpretation to the sentence (e.g. in "The boy 
love the girl" the subject can be considered plural 
- missing "s" in "boy" - or singular - missing "s" 
in "love"); this can also happen for correct sen 
tences (see the last example in the section on 
CONJUNCTIONS). The current version of the system 
should be enhanced both by taking into account con 
textual information (which could be useful in the 
first case) and by weighing in some way the output 
of the semantic component (which, today, is catego~ 
ical: yes or no). 
As regards the context, the experiments we made 
on the parser refer to isolated sentences, so that 
the "pattern matching" procedure we referred to in 
the section on ELLIPSIS (see the example "John") is 
neither implemented nor designed. Our belief is that 
the two components (pattern marcher and parser) are 
quite independent each other, but we are planning 
to address also issues connected with discourse 
analysis. 
Last but not least, some problems are more 
strictly connected with the basic parser design. 
Some English sentences break a locality principle 
embodied in the first-level syntactic rules. An 
example is given by "What architect do you know who 
likes the balalaika" (see Winograd 83, pag.136). We 
are currently studying this problem, whose solution 
will involve a change in the final representation as 
well as in the rule packets. 
The current version of the parser, that runs 
on a VAX-II/780 under the UNIX operating system and 
is implemented in FRANZ LISP, includes the mecha 
nisms for detecting and recovering the lexical, 
agreement, and word ordering errors, whereas the 
"extra cases", in the sense explained above, are 
currently being implemented. 
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539 
