An adjective clause is a dependent clause that modifies a noun or pronoun in an independent clause. It uses pronouns like who, whom, whose, which, that to connect the dependent clause to the independent clause. Common pronoun patterns in adjective clauses include using "who" for people as subjects, "whom" or "that/which" for people as objects, and "whose" to modify possession. Adjective clauses add descriptive information about a person, place, or thing.