The defining or restrictive pronoun is that; use when introducing nonparenthetic clauses; don’t set these clauses off with commas. For example: She works in the office that was remodeled. The non-defining or non-restrictive pronoun is which; use when introducing parenthetic clauses; set these clauses off with commas. For example: The book, which was published in 1996, won a Caldecott Medal.
- Use who for references to people and to animals with names: The woman who wrote the book will autograph copies or The man, who wrote a book a few years ago, is a professor.