Ensuring Pronoun-Antecedent Agreement
The word that a pronoun replaces is called its antecedent. If the antecedent is singular, the pronoun must be singular. If the antecedent is plural, the pronoun must be plural.
Four friends recorded their first song.
(Because the noun "friends" is plural, the pronoun "their" must be plural.)
Eduardo tested his vocals.
(Because "Eduardo" is singular, the pronoun "his" must be singular.)
The pronoun should also have the same person (first, second, or third) as its antecedent.