Solution
You have to add an extra argument to the following predicates:
np, vp, det,
n, pn, pro,
vi, vt, and any other verb types that
you might have.
Subject NPs should agree in their number with the verb phrase.
s --> np(subj,NUM), vp(NUM).
Determiner should agree in their number with the noun. The number
of the noun determines the number of the NP.
np(_,NUM) --> det(NUM), n(NUM).
The number of the proper name or pronoun determines the number of
the NP.
np(_,NUM) --> pn(NUM).
np(CASE,NUM) --> pro(CASE,NUM).
This is a new rule. Plural nouns can occur without determiner.
np(_,pl) --> n(pl).
The number of the VP is determined by the number of the verb. The
number of object NPs is independent of the number of the verb.
vp(NUM) --> vi(NUM).
vp(NUM) --> vt(NUM), np(obj,_).
The determiner a can only be used for singular noun
phrases, the determiner the can be used for singular as
well as for plural noun phrases.
det(sing) --> [a].
det(_) --> [the].
The lexicon specifies whether a noun, proper name or pronoun is
singular, plural, or both (as, for example, yakuza).
n(sing) --> [bride].
n(pl) --> [brides].
n(sing) --> [nurse].
n(pl) --> [nurses].
n(_) --> [yakuza].
n(sing) --> [whiskey].
pn(sing) --> [bill].
pn(sing) --> [gogo].
pro(subj,sing) --> [he].
pro(subj,sing) --> [she].
pro(obj,sing) --> [him].
pro(obj,sing) --> [her].
pro(subj,pl) --> [they].
pro(obj,pl) --> [them].
The lexicon also specifies whether a verb form is singular or
plural.
vi(sing) --> [whistles].
vi(pl) --> [whistle].
vi(sing) --> [fights].
vi(pl) --> [fight].
vt(sing) --> [drinks].
vt(pl) --> [drink].
vt(sing) --> [kills].
vt(pl) --> [kill].