Roughly speaking, idioms are expressions whose meaning cannot be
completely understood from the meanings of the component parts. For
example, whereas it is possible to work out the meaning of (
a) on
the basis of knowledge of English grammar and the meaning of words, this
would not be sufficient to work out that (
b) can mean something
like `If Sam dies, her children will be rich'. This is because
kick the bucket is an idiom.
The problem with idioms, in an MT context, is that it is not usually possible to translate them using the normal rules. There are exceptions, for example take the bull by the horns (meaning `face and tackle a difficulty without shirking') can be translated literally into French as prendre le taureau par les cornes, which has the same meaning. But, for the most part, the use of normal rules in order to translate idioms will result in nonsense. Instead, one has to treat idioms as single units in translation.
In many cases, a natural translation for an idiom will be a single
word --- for example, the French word mourir
(`die') is a possible translation for kick the bucket. This
brings out the similarity, which we noted above, with lexical
holes of the kind shown in (
).
Lexical holes and idioms are frequently instances
of word 
phrase translation. The difference is that
with lexical hole s, the
problem typically arises when one translates from the language with
the word into the language that uses the phrase, whereas with idioms,
one usually gets the problem in translating from the language that has
the idiom (i.e. the phrase) into the language which uses a single
word. For example, there is no problem in translating I do not
know the solution literally into French --- the
result is perfectly understandable. Similarly, there is no problem in
translating mourir `literally' into English (as die)
--- one is not forced to use the idiom kick the bucket.
In general, there are two approaches one can take to the treatment of idioms. The first is to try to represent them as single units in the monolingual dictionaries. What this means is that one will have lexical entries such as kick_the_bucket. One might try to construct special morphological rules to produce these representations before performing any syntactic analysis --- this would amount to treating idioms as a special kind of word, which just happens to have spaces in it. As will become clear, this is not a workable solution in general. A more reasonable idea is not to regard lexical lookup as a single process that occurs just once, before any syntactic or semantic processing, but to allow analysis rules to replace pieces of structure by information which is held in the lexicon at different stages of processing, just as they are allowed to change structures in other ways. This would mean that kick the bucket and the non-idiomatic kick the table would be represented alike (apart from the difference between bucket and table) at one level of analysis, but that at a later, more abstract representation kick the bucket would be replaced with a single node, with the information at this node coming from the lexical entry kick_the_bucket. This information would probably be similar to the information one would find in the entry for die.
In any event, this approach will lead to translation rules saying something like the following, in a transformer or transfer system (in an interlingual system , idioms will correspond to collections of concepts, or single concepts in the same way as normal words).
in_fact => en_fait in_view_of => étant_donné kick_the_bucket => mourir kick_the_bucket => casser_sa_pipe
The final example shows that one might, in this way, be able to
translate the idiom kick the bucket into the equivalent
French idiom casser sa pipe --- literally `break
his/her pipe'. The overall translation process is illustrated in
Figure 
.
The second approach to idioms is to treat them with special rules that change the idiomatic source structure into an appropriate target structure. This would mean that kick the bucket and kick the table would have similar representations all through analysis . Clearly, this approach is only applicable in transfer or transformer systems, and even here, it is not very different from the first approach --- in the case where an idiom translates as a single word, it is simply a question of where one carries out the replacement of a structure by a single lexical item, and whether the item in question is an abstract source language word such as kick_the_bucket or a normal target language word (such as mourir).
One problem with sentences which contain idioms is that they are typically ambiguous , in the sense that either a literal or idiomatic interpretation is generally possible (i.e. the phrase kick the bucket can really be about buckets and kicking). However, the possibility of having a variety of interpretations does not really distinguish them from other sorts of expression. Another problem is that they need special rules (such as those above, perhaps), in addition to the normal rules for ordinary words and constructions. However, in this they are no different from ordinary words, for which one also needs special rules. The real problem with idioms is that they are not generally fixed in their form, and that the variation of forms is not limited to variations in inflection (as it is with ordinary words). Thus, there is a serious problem in recognising idioms.
This problem does not arise with all idioms. Some are completely frozen forms whose parts always appear in the same form and in the same order. Examples are phrases like in fact, or in view of. However, such idioms are by far the exception. A typical way in which idioms can vary is in the form of the verb, which changes according to tense , as well as person and number. For example, with bury the hatchet (`to cease hostilities and becomes reconciled', one gets He buries/buried/will bury the hatchet, and They bury/buried/shall bury the hatchet. Notice that variation in the form one gets here is exactly what one would get if no idiomatic interpretation was involved --- i.e. by and large idioms are syntactically and morphologically regular --- it is only their interpretations that are surprising.
A second common form of variation is in the form of the possessive pronoun in expressions like to burn one's bridges (meaning `to proceed in such a way as to eliminate all alternative courses of action'). This varies in a regular way with the subject of the verb:
bridges.
bridges.
In other cases, only the syntactic category of an element in an idiom can be predicted. Thus, the idiom pull X's leg (`tease') contains a genitive NP, such as Sam's, or the king of England's. Another common form of variation arises because some idioms allow adjectival modifiers. Thus in addition to keep tabs on (meaning observe) one has keep close tabs on (`observe closely'), or put a political cat among the pigeons (meaning `do or say something that causes a lot of argument politically'). Some idioms appear in different syntactic configurations, just like regular non-idiomatic expressions. Thus, bury the hatchet appears in the passive, as well as the active voice.
Of course, not all idioms allow
these variations (e.g. one cannot passivize kick the bucket
meaning `die'), and, as noted, some do not allow any variation in
form. But where variation in form is allowed, there is clearly a
problem. In particular, notice that it will not be possible to
recognise idioms simply by looking for sequences of particular words
in the input. Recognising some of these idioms will require a rather
detailed syntactic analysis . For example, despite the variation in
form for bury the hatchet, the idiomatic interpretation only
occurs when the hatchet is always DEEP OBJECT of bury.
Moreover, the rules that translate idioms
or which replace them by single lexical items may have to be rather
complex. Some idea of this can be gained from considering what must
happen to pull Sam's leg in order to produce something
like equivalent to tease Sam, or the French translation involving
taquiner (`tease'), cf. Figure . This figure
assumes the input and output of transfer are representations of
grammatical relations, but the principles are the same if semantic
representations are involved, or if the process involves reducing
pull X's leg to a single word occurs in English analysis.
Rather different from idioms are expressions like those in
(
), which are usually referred to as collocations . Here the
meaning can be guessed from the meanings of the parts. What is not
predictable is the particular words that are used.
For example, the fact that we say rancid butter, but not * sour butter, and sour cream, but not * rancid cream does not seem to be completely predictable from the meaning of butter or cream, and the various adjectives. Similarly the choice of take as the verb for walk is not simply a matter of the meaning of walk (for example, one can either make or take a journey).
In what we have called linguistic knowledge (LK) systems, at least, collocations can potentially be treated differently from idioms. This is because for collocations one can often think of one part of the expression as being dependent on, and predictable from the other. For example, one may think that make, in make an attempt has little meaning of its own, and serves merely to `support' the noun (such verbs are often called light verbs, or support verbs). This suggests one can simply ignore the verb in translation, and have the generation or synthesis component supply the appropriate verb. For example, in Dutch , this would be doen, since the Dutch for make an attempt is een poging doen (`do an attempt').
One way of doing this is to have analysis replace the lexical verb (e.g. make) with a `dummy verb' (e.g. VSUP). This can be treated as a sort of interlingual lexical item, and replaced by the appropriate verb in synthesis (the identity of the appropriate verb has to be included in the lexical entry of nouns, of course --- for example, the entry for poging might include the feature support_verb=doen. The advantage is that support verb constructions can be handled without recourse to the sort of rules required for idioms (one also avoids having rules that appear to translate make into poging `do').
Of course, what one is doing here is simply recording, in each lexical entry, the identity of the words that are associated with it, for various purposes --- e.g. the fact that the verb that goes with attempt is make (for some purposes, anyway). An interesting generalisation of this is found in the idea of lexical functions . Lexical functions express a relation between two words. Take the case of heavy smoker, for example. The relationship between heavy and smoker is that of intensification, which could be expressed by the lexical function Magn as follows, indicating that the appropriate adjective for English smoker is heavy, whereas that for the corresponding French word fumeur is grand (`large') and that for the German word Raucher is stark (`strong').
(English) Magn(smoker) = heavy (French) Magn(fumeur) = grand (German) Magn(Raucher) = stark
If one wants to translate heavy smoker into French , one needs to map smoker into fumeur, together with the information that fumeur has the lexical function Magn applied to it, as in English. It would be left to the French synthesis module to work out that the value Magn(fumeur) = grand, and insert this adjective appropriately. Translation into German is done in the same way.
