Pronouns should agree with their antecedents in number and gender.

The pronouns he, him, and his are used when the antecedent is singular and in the masculine gender.

The pronouns she, her, and hers are used when the antecedent is singular and in the feminine gender.

The pronouns it and its are used when the antecedent is singular and is in the common gender that refer to animals and in the neuter gender.

The pronouns they, their, them, and theirs refer to plural antecedents in any gender.

The pronouns we, us, our, and ours refer to plural antecedents including the speaker. These also refer to persons in masculine, feminine, and common genders.