Verbs

Verbs are the other part of speech in the XTAG grammar that can assign case. Because XTAG does not distinguish INFL and VP nodes, verbs must provide case assignment on the subject position in addition to the case assigned to their NP complements. Assigning case to NP complements is handled by building the case values of the complements directly into the tree that the case assigner (the verb) anchors. Figures 4.5(a) and 4.5(b) show an S tree^4.4 that would be anchored^4.5 by a transitive and ditransitive verb, respectively. Note that the case assignments for the NP complements are already in the tree, even though there is not yet a lexical item anchoring the tree. Since every verb that selects these trees (and other trees in each respective subcategorization frame) assigns the same case to the complements, building case features into the tree has exactly the same result as putting the case feature value in each verb's lexical entry.

    

Figure 4.5: Case assignment to NP arguments

(a)		(b)

The case assigned to the subject position varies with verb form. Since the XTAG grammar treats the inflected verb as a single unit rather than dividing it into INFL and V nodes, case, along with tense and agreement, is expressed in the features of verbs, and must be passed in the appropriate manner. The trees in Figure 4.6 show the path of linkages that joins the <assign-case> feature of the V to the <case> feature of the subject NP. The morphological form of the verb determines the value of the <assign-case> feature. Figures 4.6(a) and 4.6(b) show the same tree^4.6 anchored by different morphological forms of the verb sing, which give different values for the <assign-case> feature.

  

Figure 4.6: Assigning case according to verb form

(a)		(b)

  

Figure 4.7: Proper case assignment with auxiliary verbs

(a)		(b)

The adjunction of an auxiliary verb onto the VP node breaks the <assign-case> link from the main V, replacing it with a link from the auxiliary verb instead.^4.7 The progressive form of the verb in Figure 4.6(b) has the feature-value <assign-case>=none, but this is overridden by the adjunction of the appropriate form of the auxiliary word be. Figure 4.7(a) shows the lexicalized auxiliary tree, while Figure 4.7(b) shows it adjoined into the transitive tree shown in Figure 4.6(b). The case value passed to the subject NP is now nom (nominative).