While both which and that can be used in other constructions, the confusion usually arises when they are being used as relative pronouns to introduce adjective (or relative) clauses. In the examples below, we have bracketed the adjective clauses. (Remember that a clause is simply a group of words containing a subject and a verb):
1. Our house [that has a red door and green shutters] needs painting.
2. Our house, [which has a red door and green shutters], needs painting.
3. The classrooms [that were painted over the summer] are bright and cheerful.
4. The classrooms, [which were painted over the summer], are bright and cheerful.
In all four cases, the adjective clause tells us something about either the house or the classrooms, but the choice of which or that changes the way we should read each sentence.
The rule of thumb, then, is that which clauses are nonrestrictive (nonessential) while that clauses are restrictive (essential). Nonrestrictive clauses and phrases are set off from the rest of a sentence by a pair of commas (as in our examples above) or by a single comma if they come at the end of the sentence. (Example: "I took a vacation day on my birthday, which happened to fall on a Monday this year.")
Which pronoun--which or that--belongs in each blank below?
1. Carlos gave Maria a study guide for material ________ was going to be on the test.
2. Carlos gave Maria notes from chapters 3 through 7 _________ were going to be on the test.
3. Mark and Sarah took their children on every vacation _________ they took to the coast.
4. The teachers gave awards to all paintings ________ showed originality.