Using "That" or "Which?"

That: Should be used restrictively to narrow a category or identify a particular item being talked about.

Example: -This is the house that Jack built.

-There are a lot of grammar mistakes that make you look bad.

Which: Should be used non-restrictively. Not to narrow a class or identify a particular item but to add something about an item already identified. Almost always preceded by a comma, a parenthesis, or a dash.

Example: -This is the house that Jack built, which he later sold.

-Jill received a good grade on her paper, which she desperately needed to pass the class.


Exception: “Which” should be used restrictively only when it is preceded by a
preposition.

Example: -The room in which he ate his dinner.