Pronouns have to agree in person, number and gender with the words they refer to (called their antecedents). When indefinite pronouns are used as antecedents, special problems can arise.
(For basic guidelines on pronoun agreement, see Pronouns have to agree. For more information on and examples of indefinite pronouns, read Parts of Speech: Indefinite Pronouns.)
Certain indefinite pronouns (e.g., anything, each, everyone, other) are always singular. They are also usually inclusive—that is, they refer to both males and females. When these pronouns are used as inclusive antecedents, the pronouns referring to them must be singular and inclusive:
If anyone here plans to enter the contest, he or she must sign the list.
[he or she is singular and inclusive to agree with anyone]
Everybody in the cast has already put on his or her costume.
[his or her is singular and inclusive to agree with everybody]
BUT
Everyone on the girls' team is wearing her uniform.
[her is feminine singular to agree with everyone, since all are girls]
Note: In speech and informal writing, to avoid he or she and its variations, people often use the inclusive plural pronoun they or one of its forms with a singular antecedent:
If anyone here plans to enter the contest, they must sign the list.
In formal writing, this solution is not yet widely accepted and should be avoided.
When plural indefinite pronouns (e.g., both, few, many, several) are used as antecedents, the pronouns referring to them must be plural:
Both passed their certification exam.
[their is plural to agree with both]
Jolanta tasted several of the desserts and found them delicious.
[them is plural to agree with several]
Some indefinite pronouns (e.g., all, most, none, some) can be either singular or plural. Usually, these pronouns are followed by a prepositional phrase containing the noun to which they refer. To figure out whether the indefinite pronoun antecedent is singular or plural, look at the noun in the phrase:
All of the paint has lost its shine.
[all refers to paint and is singular, so use the singular pronoun its]
All of the dogs had their walks in the morning.
[all refers to dogs and is plural, so use the plural pronoun their]
None of the music lived up to its reputation.
[none refers to music and is singular]
None of the candidates did well in their interviews.
[none refers to candidates and is plural]
Note: In the examples above, the pronoun none means "not any." This pronoun can also be used with the singular meaning "not a single one." Writers who want to emphasize this singular meaning can choose the pronoun his or her even when none is followed by a plural noun:
None of the candidates did well in his or her interview.