Much More about Pronouns -- American English - Page 14

Indefinite pronouns also present a problem in number. Some of them are always singular. Some are always plural, and some may be either singular or plural.

The following are always singular. A pronoun that is used in place of these indefinite pronouns must also be singular.

Note the following examples. They show the proper agreement between the pronoun and antecedent when the antecedent is singular.

Neither of the men had his tools. (not their)
If anyone wants a pencil, he can buy one here. (not they)
One likes to do what he can do well. (not they)
Someone left his book in his locker. (not their)