16.3.1 Agreement

As discussed in Ch. 12, agreement phenomena show up in a number of different places in English. This section illustrates how unification can be used to capture the two main types of English agreement phenomena: subject-verb agreement and determiner-nominal agreement. We will use the following ATIS sentences as examples throughout this discussion to illustrate these phenomena.

(16.8) This flight serves breakfast.

(16.9) Does this flight serve breakfast?

(16.10) Do these flights serve breakfast?

Notice that the constraint used to enforce SUBJECT-VERB agreement given above is deficient in that it ignores the PERSON feature. The following constraint which makes use of the AGREEMENT feature takes care of this problem.

S $ \rightarrow $ NP VP

$$ \langle N P\ A G R E E M E N T\rangle=\langle V P\ A G R E E M E N T\rangle $$

Examples 16.9 and 16.10 illustrate a minor variation on SUBJECT-VERB agreement. In these yes-no-questions, the subject NP must agree with the auxiliary verb, rather than the main verb of the sentence, which appears in a non-finite form. This agreement constraint can be handled by the following rule.

$ S \rightarrow Aux NP VP $

$$ \langle A u x~A G R E E M E N T\rangle=\langle N P~A G R E E M E N T\rangle $$

Agreement between determiners and nominals in noun phrases is handled in a similar fashion. The basic task is to permit the expressions given above, but block the unwanted *this flights and *those flight expressions where the determiners and nominals clash in their NUMBER feature. Again, the logical place to enforce this constraint is in the grammar rule that brings the parts together.

$$ NP\ \to\operatorname{Det}\operatorname{Nominal} $$

$$ \langle D e t\ A G R E E M E N T\rangle=\langle N o m i n a l\ A G R E E M E N T\rangle $$

$$ \langle N P\ A G R E E M E N T\rangle=\langle N o m i n a l\ A G R E E M E N T\rangle $$

This rule states that the AGREEMENT feature of the Det must unify with the AGREEMENT feature of the Nominal, and moreover, that the AGREEMENT feature of the NP must also unify with the Nominal.

Having expressed the constraints needed to enforce subject-verb and determiner-nominal agreement, we must now fill in the rest of the machinery needed to make these constraints work. Specifically, we must consider how the various constituents that take part in these constraints (the Aux, VP, NP, Det, and Nominal) acquire values for their various agreement features.

We can begin by noting that our constraints involve both lexical and non-lexical constituents. The simpler lexical constituents, Aux and Det, receive values for their respective agreement features directly from the lexicon as in the following rules.

$$ Aux \longrightarrow do $$

$$ \langle A u x A G R E E M E N T N U M B E R\rangle=p l $$

$$ \langle A u x A G R E E M E N T P E R S O N\rangle=3r d $$

$$ Aux\to{d o e s} $$

$$ \langle A u x~A G R E E M E N T~N U M B E R\rangle=s g $$

$$ \langle A u x A G R E E M E N T P E R S O N\rangle=3r d $$

$$ Det\ \to\ this $$

$$ \langle D e t{\mathrm{~A G R E E M E N T~N U M B E R}}\rangle=s g $$

Det → these

$$ \langle D e t\ A G R E E M E N T\ N U M B E R\rangle=p l $$

Returning to our first S rule, let us first consider the AGREEMENT feature for the VP constituent. The constituent structure for this VP is specified by the following rule.

$$ VP\to Verb NP $$

It seems clear that the agreement constraint for this constituent must be based on its constituent verb. This verb, as with the previous lexical entries, can acquire its agreement feature values directly from lexicon as in the following rules.

$$ Verb\rightarrow serve $$

$$ \langle V e r b~A G R E E M E N T~N U M B E R\rangle=p l $$

Verb $ \rightarrow $ serves

$$ \langle V e r b~A G R E E M E N T~N U M B E R\rangle=s g $$

$$ \langle{V e r b~A G R E E M E N T~P E R S O N}\rangle=3r d $$

All that remains is to stipulate that the agreement feature of the parent VP is constrained to be the same as its verb constituent.

$$ VP\to Verb NP $$

$$ \langle V P{~A G R E E M E N T}\rangle=\langle V e r b{~A G R E E M E N T}\rangle $$

In other words, non-lexical grammatical constituents can acquire values for at least some of their features from their component constituents.

The same technique works for the remaining NP and Nominal categories. The values for the agreement features for these categories are derived from the nouns flight and flights.

Noun $ \rightarrow $ flight

$ \langle Noun\ AGREEMENT\ NUMBER\rangle = sg $

Noun $ \rightarrow $ flights

$$ \langle N o u n A G R E E M E N T N U M B E R\rangle=p l $$

Nominal features can be constrained to have the same values as their constituent nouns.

Nominal $ \rightarrow $ Noun

$$ \langle Nominal~AGREEMENT\rangle=\langle Noun~AGREEMENT\rangle $$

Note that this section has only scratched the surface of the English agreement system, and that the agreement system of other languages can be considerably more complex than English.