Singular and Plural Pronoun
Antecedents:

An antecedent is a word that appears early
in a sentence or paragraph, one that later words may refer
to or replace. Pronouns should have a noun or another pronoun
that serves as their antecedent.
For instance, in the following sentence, the noun carpenter
is an antecedent for the pronoun his.
The carpenter needs his
tools.
We could theoretically write, "The carpenter needs
the carpenter's tools," but that creates a lot of
clutter. The pronoun stands in the place of carpenter
so we don't have to keep repeating the word. Since in this
sentence the subject is a single carpenter, the pronoun
must be singular to match the singular noun in the
sentence above. The verb needs
is a singular verb also, matching the number of the noun.
However, if the sentence contained multiple carpenters,
the plural antecedent would require a plural pronoun.
The seven carpenters need
their tools.
Now, the antecedent carpenters is plural, so it must take
a plural pronoun. The verb need
is a plural verb here, also matching the number of the noun.
The material above seems simple enough, but three tricky
areas can cause problems.
First, there are indefinite pronouns that do not refer
a specific person or object--some of them are considered
singular and others are considered plural. You
can read about them here. Second, a writer can shift
between pronouns. You
can read about this problem here. Finally, a sloppy
writer can confuse the reader by creating sentences in which
it is unclear who or what the pronoun refers to. You can
read about that problem
here.