An adjective clause is a subordinate (dependent)` clause used as an adjective. Like single-word adjectives, adjective clauses describe and modify nouns.
The following relative pronouns introduce adjective clauses.
Who refers only to persons. The man is a police officer. He lives next door. The man who lives next door is a police officer.
Whom is the objective form of who and refers only to persons. He is one police officer. I respect him very much. He is one police officer whom I respect very much.
Which refers to animals. Last week someone hit his son’s dog. The dog was only a puppy. Last week someone hit his son’s dog, which was only a puppy. refers to things. The driver ran through a stop sign. The stop sign is on the corner. The driver ran through a stop sign, which is on the corner. refers to groups of people (the audience, crowd, class). Officer Smith often lectures to high school classes. These classes are learning safe driving principles. Officer Smith often lectures to high school classes, which are learning safe driving principles. That refers to persons. The police officers respect him, too. Officer Smith works with them. The police officers that Officer Smith works with respect him, too. refers to animals. He was very happy with the new puppy. I gave him the puppy last night. He was very happy with the new puppy that I gave him last night. refers to things. I also gave his son a book. The book tells about caring for pets. I also gave his son a book that tells about caring for pets. Whose indicates possession and is used for animals. The first puppy was six weeks old. His coat was really beautiful. The first puppy, whose coat was really beautiful, was six weeks old. used for people. He is a little boy. His love for animals is very strong. He is a little boy whose love for animals is very strong.