As we've seen (in Subordination with Adjective Clauses), an adjective clause is a group of words that works like an adjective to modify a noun. Here we'll focus on the five relative pronouns that are used in adjective clauses.
An adjective clause usually begins with a relative pronoun: a word that relates the information in the adjective clause to a word or a phrase in the main clause.
The most common adjective clauses begin with one of these relative pronouns: who, which, and that. All three pronouns refer to a noun, but who refers only to people and which refers only to things. That may refer to either people or things.
Two other relative pronouns used to introduce adjective clauses are whose (the possessive form of who) and whom (the object form of who). Whose begins an adjective clause that describes something that belongs to or is a part of someone or something mentioned in the main clause:
The ostrich, whose wings are useless for flight, can run faster than the swiftest horse.Whom stands for the noun that receives the action of the verb in the adjective clause:
Anne Sullivan was the teacher whom Helen Keller met in 1887.Notice that in this sentence Helen Keller is the subject of the adjective clause, and whom is the object. Put another way, who is equivalent to the subject pronouns he, she, or they in a main clause; whom is equivalent to the object pronouns him, her, or them in a main clause.