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Subsections


   
Prepositions

   
Introduction

Prepositions have not been much studied in traditional lexical semantics [Zel93], [Tru92], [VanD94], [Lin97] compared to the large amount of work devoted to verbs and to nouns. They nevertheless play a very important role in semantics and have many connections with most of the other syntactic categories. They have been, however, extensively studied in psycho-linguistics, in particular spatial prepositions.

   
Main organization of prepositions

Prepositions occur in the following three main syntactic situations:

When dealing with prepositions, a clear distinction appears between semantically full and semantically empty prepositions.

Let us now examine a simple semantic classification for prepositions. Note first that prepositions are highly polysemous; they are also subject to many metaphorical extensions, for example spatial preopositions have for most of them a metaphorical extension to abstract entities, viewed as locations (e.g. against a wall $\rightarrow$ against some principles). We can however tentatively propose the following general classification, where some items may overlapp on different ontological domains (e.g. time and space):

Prepositions across languages

The use of prepositions can, in general, be specified quite unambiguously. For example, for an event occuring after a certain date, the prepositions after, nach(dem), na(dat) après, despuès will be respectively used for English, German, Dutch, French and Spanish, if the action takes place before the date, then we respectively have before, (be)vor, voor(dat), avant, antes.

Prepositions do not have often direct correspondences across languages. For example if S is an event, we have the following preposition lexicalizations depending on the duration and the type of event:

1.
simple point: at, bei, bij, à (English, German, Dutch, French),
2.
S has a duration, and happened once in the past: when/as, als, toen, lors de; als becomes wie in case of present tense in German,
3.
otherwise: when/as, wenn, als/wanneer, lors de.
¿From a different perspective around translates in German as gegen, um, at translates as um, zu, bei, an, in and by, until translate as bis.

Similarly, verbs may select very different types of prepositions in two different languages:
dream about someone,
réver de/à quelqu'un,
soñar con alguien.
(Spanish: to dream with someone)
Whereas about and de have about equivalent meanings, con is completely different. There are many divergences of this type which make machine translation quite difficult.

In the context of a multilingual system, the need for assigning some sort of semantics to modifying prepositional phrases may be illustrated by these two facts

Italian per incorporates both the meanings of Spanish para and por: without any kind of semantic information it is not possible to decide which translation must be chosen, as may be seen in the following examples. The first set corresponds to benefactive per and the second to causative per:
1. Ha scritto una lettera per la sua fidanzata
Ha escrito una carta para su novia
(He) has written a letter for her girlfriend
2. Ha scritto una lettera per paura
Ha escrito una carta por miedo
(He) has written a letter out of fear.

   
Prepositions and Semantic Relations for Modifiers

The work reported here has mainly been developed within Eurotra, 3.9.1.

Semantic Relations for Modifiers : MODSR An experiment on Semantic Relations for Modifiers was carried out in the context of the Eurotra project by some language groups during 1989 and was afterwards adopted as legislation by the rest of the groups.

>From the examples above, it seems clear that a mere lexical typification of the preposition (like benefactive or causative) is not sufficient to perform the disambiguation, due to the polysemous nature of most prepositions. A calculation of the semantic value ( or relation ) of the whole prepositional phrase is needed. Thus, the semantic relation (or modsr value) of a PP of the form: P + NP has to be calculated on the basis of the lexical value of the preposition P combined with the lexical semantic value of the NP. Thus, for instance, the preposition with may have several semantic values, depending on the NP that follows it:

 
        PP[ with + NP (sem=hum)] =>  PP[modsr=accompaniment]                
        
        PP[ with + NP (sem=non-hum)] => PP[modsr=instrument]

It appears that some set of lexical semantic features (LSF) for nouns is needed, but since the process of calculation is internal to each language module, each of them is free to use its own set, which may have different degrees of granularity. On the other hand, the set of modsr labels belongs to the Eurotra Interface Structure (IS) and thus is shared by all languages. Here follows the list of modsr values that was agreed upon. They are divided into two big groups :

This distinction is justified because, from a semantic point of view, the relation between the modifier and the head is of a different type in both cases, as sustained by [Pol87]. Also , from a syntactic point of view, the choice of the preposition for modifiers of nominal heads is much more restricted, especially in the Romance languages where the most frequent is of (e in French, Spanish, Portuguese and Catalan, and di in Italian). We could say that nonpredicative (or nominal) prepositional modifiers behave like adjectives while predicative prepositional modifiers behave like adverbs. Predicative nouns are a bit special because they combine a verbal and a nominal nature.

   
Relations with other areas of lexical semantics

The relations that prepositions have with other elements of lexical semantics are the following:

   
Prepositions in LKB

Prepositions being a closed set of words are usaully described in depth in most lexical knowledge bases. They can be grouped into families as suggested above in 2.9.1. They usually share a number of properties, but are not in general included into major taxonomies.

   
Prepositions in Applications

As already shown above, prepositions may be quite difficult to translate in MT applications 4.1, and at that level, lexical semantics is of much use to help disambiguate and select the right prepositions 3.9, 3.10, 3.7 5.3.

Another important role of prepositions is to contribute to solving ambiguities in PP-attachment. This is also a complex task, but the semantics of prepositions can be used to decide whether the PP is a verb complement (argument or modifier) or a noun complement.

Finally, prepositions convey a quite important semantic load, in spite of their high polysemic nature. They are in particular important to identify the semantics of modifiers. From that point of view, they are of much use in information retrieval systems for the extraction on information 4.3, 4.2 in modifiers (e.g. when predicate-argument structures are used, possibly combined with thematic roles identifying the roles of NPs or PPs).



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Next: Adverbs Up: Linguistic aspects of lexical Previous: Adjectives
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