Subcategorization

In Chapter 8, we augmented our category labels to represent different kinds of verbs, and used the labels IV and TV for intransitive and transitive verbs respectively. This allowed us to write productions like the following:

Although we know that IV and TV are two kinds of V, they are just atomic non-terminal symbols in a CFG and are as distinct from each other as any other pair of symbols. This notation doesn't let us say anything about verbs in general; e.g., we cannot say "All lexical items of category V can be marked for tense," since walk, say, is an item of category IV, not V. So, can we replace category labels such as TV and IV by V along with a feature that tells us whether the verb combines with a following NP object or whether it can occur without any complement?

A simple approach, originally developed for a grammar framework called Generalized Phrase Structure Grammar (GPSG), tries to solve this problem by allowing lexical categories to bear a SUBCAT feature, which tells us what subcategorization class the item belongs to. In contrast to the integer values for SUBCAT used by GPSG, the example here adopts more mnemonic values, namely intrans, trans, and clause:

(30) VP[TENSE=?t, NUM=?n] -> V[SUBCAT=intrans, TENSE=?t, NUM=?n] VP[TENSE=?t, NUM=?n] -> V[SUBCAT=trans, TENSE=?t, NUM=?n] NP VP[TENSE=?t, NUM=?n] -> V[SUBCAT=clause, TENSE=?t, NUM=?n] SBar

V[SUBCAT=intrans, TENSE=pres, NUM=sg] -> 'disappears' | 'walks' V[SUBCAT=trans, TENSE=pres, NUM=sg] -> 'sees' | 'likes' V[SUBCAT=clause, TENSE=pres, NUM=sg] -> 'says' | 'claims'

V[SUBCAT=intrans, TENSE=pres, NUM=pl] -> 'disappear' | 'walk' V[SUBCAT=trans, TENSE=pres, NUM=pl] -> 'see' | 'like' V[SUBCAT=clause, TENSE=pres, NUM=pl] -> 'say' | 'claim'

V[SUBCAT=intrans, TENSE=past] -> 'disappeared' | 'walked' V[SUBCAT=trans, TENSE=past] -> 'saw' | 'liked' V[SUBCAT=clause, TENSE=past] -> 'said' | 'claimed'

When we see a lexical category like V[SUBCAT=trans], we can interpret the SUBCAT specification as a pointer to a production in which V[SUBCAT=trans] is introduced as the head child in a VP production. By convention, there is a correspondence between the values of SUBCAT and the productions that introduce lexical heads. On this approach, SUBCAT can appear only on lexical categories; it makes no sense, for example, to specify a SUBCAT value on VP. As required, walk and like both belong to the category V. Nevertheless, walk will occur only in VPs expanded by a production with the feature SUBCAT=intrans on the righthand side, as opposed to like, which requires a SUBCAT=trans.

In our third class of verbs in (30), we have specified a category SBar. This is a label for subordinate clauses, such as the complement of claim in the example You claim that you like children. We require two further productions to analyze such sentences:

The resulting structure is the following.

An alternative treatment of subcategorization, due originally to a framework known as categorial grammar, is represented in feature-based frameworks such as PATR and Head-driven Phrase Structure Grammar. Rather than using SUBCAT values as a way of indexing productions, the SUBCAT value directly encodes the valency of a head (the list of arguments that it can combine with). For example, a verb like put that takes NP and PP complements (put the book on the table) might be represented as (33):

This says that the verb can combine with three arguments. The leftmost element in the list is the subject NP, while everything else—an NP followed by a PP in this case—comprises the subcategorized-for complements. When a verb like put is combined with appropriate complements, the requirements which are specified in the SUBCAT are discharged, and only a subject NP is needed. This category, which corresponds to what is traditionally thought of as VP, might be represented as follows:

Finally, a sentence is a kind of verbal category that has no requirements for further arguments, and hence has a SUBCAT whose value is the empty list. The tree (35) shows how these category assignments combine in a parse of Kim put the book on the table.

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