Pronoun-Antecedent Agreement
A pronoun must agree with its antecedent — the noun it replaces — in number and gender, and the antecedent must be unambiguous.
Case matters too: subject pronouns (I, he, who) do the acting and object pronouns (me, him, whom) receive it — test compound phrases by dropping the other person (‘Sam and me went’ fails as ‘me went’).