The verb in an adjective clause must agree in number with the noun it refers to. If the noun is singular, the verb in the adjective clause is also singular. If the noun is plural, the verb in the adjective clause is plural as well. For example, if the noun is "man" which is singular, the verb in the adjective clause is "is"; but if the noun is "men" which is plural, the verb becomes "are".

1. 12-5 Singular and Plural Verbs in
AdjectiveClauses
Page 331

2. Theverbinthe
adjectiveclauses
matchesthenoun
thatitreplaces.
The verb in the adjective clause is
singular if the pronoun replaces a
singular noun.
I know the man who is sitting over
there.

3. Theverbinthe
adjectiveclauses
matchesthenoun
thatitreplaces.
The verb in the adjective clause is
singular if the pronoun replaces a
singular noun.
I know the men who are sitting over
there.