MACHINE TRANSLATION, LINGUISTICS, AND INTERLINGUA 
Petr Sgall and Jarmila Panevov~ 
Faculty of Mathematics and Physics, 
Charles University 
Malostransk4 n.25, 118 O0 Praha l, Czechoslovakia 
ABSTRACT 
An adequate, complete, and economical 
linguistic theory is necessary for MT and 
the question is whether a consistent use 
of the often unduly neglected dependency 
syntax, including a systematic description 
of topic and focus, cannot serve as a re- 
liable base for the grammar of an inter- 
lingua, or of a set of interrelated inter- 
face structures. 
i. As Slocum (198~)convinc~ly shows, the 
attitude towards translation in general, 
and therefore also towards automatic trans- 
lation in the U.S.A. never has been based 
on urgent wide-spread needs of translating 
technical texts, and mostly has not been 
connected with broad interest in theoreti- 
cal background. Outside the U.S.A., with 
the exception of G.E.T.A., Grenoble, the 
research had the character of scattered 
projects carried out by relatively small 
groups; only in the recent years the EUROTRA 
project and, especially, the two Japanese 
projects bring some hope as for the possi- 
bility of sufficiently concentrated re- 
search. 
The question whether linguistics is able 
to offer a reliable theoretical basis for 
MT cannot be answered in a qualified way 
without examining such linguistically based 
systems as Garvin~s'fulcrum" approach {which 
was abolished on external grounds, after 
the unfortunate ALPAC report~ or the sys- 
tems formulated by Kulagina and Apresyan. 
Certain features of their frameworks, as 
well as of Vauquois" (1975; Vauquois and 
Boitet, 1985~ are more closely connected to 
classical structural linguistics than is 
the case with other MT systems. Also in 
Prague, the research group of MT and formal 
linguistics at Charles University has de- 
voted much effort (starting at the end of 
the 1950"s~ to identify the positive results 
of classical European linguistics and to 
reformulate them in a metalanguage that 
would make them usable in the context of 
Chomskyan (and Montaguean~ methodology and 
of automated language processing (now see 
Sgall et al., 1986~. 
2. The requirements on lin@uistic theory 
as a background for MT can be summarized as 
follows: 
(a~ Adequacy: The theory should underlie 
relatively complete descriptions reflecting 
the structure of language. Since humans dif- 
fer from computers (in freely combining fac- 
tual knowledge and other mental capacities 
with their knowledge and use of language, 
and in being able to develop their language 
while using itS, the correspondence between 
theory and its MT application cannot be im- 
mediate. The open-endedness of language 
makes it necessary to restrict the complete- 
ness of the description to a reasonably 
estimated core of language, leaving the 
~ossibly not too large~ periphery inrthe 
application to postediting, etc. 
99 
(b) Testability! It follows from (a) 
that testability also is limited: not every 
counterexample disqualifies a theory. 
(c) Economy: To be applicable, the theo- 
ry cannot be too complex. It seems necessary 
to draw a boundary line between the system 
of language on the one side, and its use or 
its semantic interpretation on the other, 
although in several respects it may be use- 
ful for the applications not to follow this 
boundary quite exactly. 
(d) Modularity: Since huge programs are 
extremely difficult to be handled (debugged, 
updated, etc.), priority will be given to 
such a theory that not only allows for a 
division of labour between the description 
of linguistic and communicative competence 
- see (c), - but makes also a cooperation 
between specialists in the different layers 
of language itself possible. 
The comparison of different approaches 
to linguistic theory as to point (a) is a 
matter of the theory itself; let us only 
note that many theories seem not to be suf- 
ficiently adequate in that they do not 
properly distinguish between the three di- 
mensions of the sentence structure (valency 
or theta roles, coordination and apposition, 
and also topic and focus, which often is 
almost altogether neglected I) and the mor- 
phological categories (tense, aspect, num- 
ber, definiteness, and so on); the latter 
occupy no immediate positions in the struc- 
ture of the sentence with its recursive 
properties, and thus it is not adequate to 
denote e.g. prepositions as if they per- 
1 The relevance of the topic-focus ar- 
ticulation for translation and for other 
aims of language comprehension can be illus- 
trated by the following examples: In the 
hallway one smokes should be distinguished 
from One smokes inthe hallway similarly as 
Few books are read by many men from Many 
men read few books. 
mitted for unlimited complementation, as 
verbs do. 
For point (b) it is important that the 
theory uses operational criteria in delim- 
iting its units and oppositions, thus re- 
presenting a suitable starting point for 
implementable application systems. 
With regard to (c), the relative gener- 
ality of the formulations used by the the- 
ory is relevant; thus e.g. Chomsky's uni- 
versal principles are relatively econom- 
ical. On the other hand, the abundance of 
nodes in the P-markers (cf. what was just 
said on point (a)) brings along the ne- 
cessity to use tree pruning and to intro- 
duce devices making it possible to find an 
orientation in the unnecessarily large 
trees. 
As for (d), it seems preferable to work 
with two levels of sentence structure and 
with a separate level of morphemic repre- 
sentations in the theory, although in the 
applications this pattern may be simplified 
(we are then aware what we have left out 
e.g. in our parser, and are able to restore 
a missing subpart, if this proves to be 
necessary, e.g. when the system is to be 
generalized to handle new kinds of texts). 
A systematic investigation into compar- 
ing different linguistic theories from 
these viewpoints has resulted in our pre- 
ference for dependency grammar, based on 
valency or theta roles (see Sgall et al., 
1986, for a detailed discussion). A depend- 
ency based linguistic description is ade- 
quate in the quoted respect \[e.g. the mor- 
phological values are denoted here by parts 
of complex node labels); the theory is ful- 
ly testable and uses operational criteria, 
and it ensures both economy \[no non-termi- 
nal symbols are present in the representa- 
tions although as man N as necessary can be 
used during the derivation procedure) and 
modularity (the underlying representations 
contain all the semantically relevant in- 
100 
formation, since also the topic-focus ar- 
ticulation is denoted here). 
Bloomfield's °exocentric ° constructions 
are often mistakenly understood as an ob- 
stacle for dependency syntax; however, as~ 
Robinson (1970):showed, they can be hanv 
dled without serious difficulties within 
dependency trees. Let us add that, if con- 
structions are analyzed in the terms of 
word classes (parts of speech), rather than 
in those of individual words, than the dis- 
tributional properties clearly show that 
e.g. your sister is a noun group (since 
e.g. Mother or syntax occur withouta deter- 
miner), to hit the ball is a verb group 
(due to to read,...), and also a sentence 
has a verb as its governor, since in I_~t 
rains no subject (Actor/Bearer) is present 
at the level of meaning (or in the under- 
lying structure). 
A formal treatment combining dependency 
syntax with a description of coordination 
and apposition, allowing for an indefinite 
number of sister nodes, was presented by 
Pl~tek et al. (1984). 
As one of the referrresofour papers has 
duly recalled, the number of publications 
concerning dependency grammar is much 
smaller than that on constituent structures, 
but the popularity of the model is not di- 
rectly relevant for its evaluation. There- 
fore it seems highly useful to notice the 
advantages of the less known model, a more 
intensive use of which might be of impor- 
tance for the further development of the 
field. 
3. An interlin@ua for MT can well be based 
on such a theory. Since the 1960"s - see 
e.g.Mel°~uk (1962), Vauquois (1962), Sgall 
(1963) - the research in this direction has 
been connected with theoretical investiga- 
tions. It has been clear that the formula- 
tion of an interlingua is a practical task, 
for the underlying units differ from one 
language to the other, so that the struc- 
ture of interlingua is based rather on the 
structural similarities (formal universals) 
of languages than on an assumed identity 
of their underlying structures, or their 
patternings of meaning. 
As for the known difficulties concerning 
e.g. the formulation of fail-soft rules or 
the presence of surface clues (see Slocum, 
1985,5; Vauquois and Boitet, 1985), it ap- 
pears that for a multilingual system of MT 
these disadvantages have to be compared 
with those present in the large number of 
binary systems which are otherwise necess- 
ary. The difference between the use of an 
interlingua and of a smaller number of 
"interfaces" (one for each language)appears 
not to be crucial. If, for a system includ- 
ing ~ languages, m among them display a 
certain opposition (that of dual versus 
plural number, or of gender with personal 
pronouns, etc.), then the degree of impor- 
tance of this opposition for the system 
depend& on the difference between n and m 
and on the importance of the languages dis- 
playing the opposition. Extremely marginal 
oppositions will probably be ignored in a 
system using interfaces as well as in one 
with an interlingua. In this case, a trans- 
lation between two languages exhibiting the 
marginal opposition will be faced with a 
similar problem as a translation from a 
"prototypical" language into a "marginal" 
one (e.g. the use of dual number will be 
determined - perhaps only for some cases - 
by contextual clues, rather than by the 
presence of dual in the input text). 
If the relative weight of such surplus 
difficulties (and resulting mistakes) is 
considerable, then it may be useful to for- 
mulate interfaces, perhaps not always in a 
one-to-one correspondence to the processed 
languages, but relating to certain groups 
of them. Certain "dialectal" differences in 
the interlingua would then be useful, each 
101 
of which would share some opposition(s) 
with a group of the processed languages. 
This may concern the differences between 
languages having and not having articles, 
verbal aspects, various moods, and so on. 
The substitution of a single interlingua by 
a set of closely related interface struc- 
tures <see VauquoM and Boitet, 1985,32; as 
for its application in the EUROTRA project, 
Johnson etal., 1985,164) perhaps is also 
important with regard to handling the se- 
mantic relationships between the lexical 
units of the languages concerned. 
This schematical view can be systemat- 
ically elaborated only on the basis of ex- 
perience with multilingual MT systems. 
4. A not quite negligible experience with 
MT systems based (at least to a great part~ 
on dependency syntax has been gained al- 
ready. The Grenoble group has used a graph 
grammar based on this approach within a sys- 
tem that is multilingual, though centred 
arotmd French (see Vauquois, 1975; Boitet 
and Nedobejkine, 1981; Vauquois and Boitet, 
1985,28f); although in this system the de- 
pendency relations are used along with a 
kind of phrase structure, the importance 
of complex node labels and of the syntactic 
relations (valency) has always been fully 
recognized. Also Nagao etal. (1985,esp.98) 
point out that dependency tree structures 
are used in their project (which certainly 
belongs to those with the best traditions 
and results); in the Eurotra system the de- 
pendency relations and the notion of "gov 
(ernor)" play an important role (see e.g. 
Johnson etal., 1985). In Prague, especially 
the English-to-Czech translation project, 
the main author of which is Kirschner (1982; 
1984), is based on a dependency description. 
5. The perspectives of MT seem to be con- 
nected with two major conditions, in addi- 
tion to the choice of an appropriate under- 
lying linguistic theory, which we discussed 
above: 
(a~ As is known, for the resolution of 
many lexical ambiguities and also for the 
identification of grammatically obligatory 
values of the target language not present 
in the input text, a MT system has to in- 
clude not only a purely linguistic descrip- 
tion. It has to be found out to what de- 
gree the practical purposes of MT can be 
achieved by systems "modelling the world" 
by such elementary means as sets of seman- 
tic features. Where means of this kind 
will be found to be insufficient, it is 
probable that neither data bases of the 
common types will do. It is then necessary 
to look for suitable kinds of knowledge 
representation systems. 
(b) The main perspective appears to be 
connected with the hope that a wider prac- 
tical application of MT will lead to a new 
situation, in which the construction of MT 
systems will no longer be a matter of 
small research groups scattered and more 
or less isolated in different countries, 
but there will emerge large-scale and well- 
-coordinated international projects based 
on the best results achieved and verified 
by widespread practical application. Under 
such new circumstances it will be possible 
not only to compile grammatically well . 
founded data on tens of thousands of lexi- 
cal units from different languages, but 
also to connect translation systems in an 
effective way with broadly based nets of 
knowledge representation. Effective ways 
of human-machine interaction can then be 
found, and the formulation of appropriate 
intermediate languag~will meet good con- 
ditions. Post-editing will certainly re~ 
main necessary, the main condition being 
that it should not be much more difficult 
than it is with human translations of 
technical texts (although other kinds of 
mistakes will prevail). 
102 
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109 
