Pronouns substitute for nouns; they are a kind of shorthand. The word a pronoun refers to is called its antecedent.
A pronoun and its antecedent agree when they are both singular or both plural.
SINGULAR
PLURAL
Multilingual: Pronoun-antecedent agreement
Exercises:
Pronoun-antecedent agreement 1
Pronoun-antecedent agreement 2
Pronoun-antecedent agreement 3
Pronoun-antecedent agreement 4