Sentential Subjects

Tree families: Ts0Vnx1, Ts0Ax1, Ts0N1, Ts0Pnx1, Ts0ARBPnx1, Ts0PPnx1, Ts0PNaPnx1, Ts0V, Ts0Vtonx1, Ts0NPnx1, Ts0APnx1, Ts0A1s1. Verbs that select sentential subjects anchor trees that have an S node in the subject position rather than an NP node. Since extraction is not possible from sentential subjects, they are implemented as substitution nodes in the English XTAG grammar. Restrictions on sentential subjects, such as the required that complementizer for indicatives, are enforced by feature values specified on the S substitution node in the elementary tree. Sentential subjects behave essentially like sentential complements, with a few exceptions. In general, all verbs which license sentential subjects license the same set of clause types. Thus, unlike sentential complement verbs which select particular complementizers and clause types, the matrix verbs licensing sentential subjects merely license the S argument. Information about the complementizer or embedded verb is located in the tree features, rather than in the features of each verb selecting that tree. Thus, all sentential subject trees have the same <mode>, <comp> and <assign-comp> values shown in Figure 8.8(a).

   

Figure: Comparison of <assign-comp> values for sentential subjects: $\alpha$s0Vnx1 (a) and sentential complements: $\beta$nx0Vs1 (b)

(a)		(b)

The major difference in clause types licensed by S-subjs and S-comps is that indicative S-subjs obligatorily have a complementizer (see examples in section 8.2). The <assign-comp> feature is used here to license a null complementizer for infinitival but not indicative clauses. <assign-comp> has the same possible values as <comp>, with the exception that the nil value is `split' into ind_nil and inf_nil. This difference in feature values is illustrated in Figure 8.8. Another minor difference is that whether but not if is grammatical with S-subjs.^8.11 Thus, if is not among the <comp> values allowed in S-subjs. The final difference from S-comps is that there are no S-subjs with <mode>=ger. As noted in footnote 4 of this chapter, gerundive complements are only allowed when there is no corresponding NP parse. In the case of gerundive S-subjs, there is always an NP parse available.