That/Which

That

Used to introduce a restrictive clause.

E.g. "I want an apple that doesn't have worms in it."

-Describes the exact apple desired, clause is restrictive, no need for commas because the clause is essential to the sentence's meaning.

Which

Used to introduce a nonrestrictive clause.

E.g. "I want an apple, which is a delicious type of fruit."

-Says nothing about the specific apple desired, the clause is "nonrestrictive," that is, it describes all apples, requires a comma before the clause because the clause is nonessential to the sentence's meaning.

A case where proper use is critical:

He likes dogs that don't bark.

He likes dogs, which don't bark.

In the first case we know that "he" likes dogs that don't bark.

In the second case, we're told two things: "he" likes dogs, and dogs don't bark.

(However, it seems to me that this distinction is so rare that we might eliminate it altogether by properly using commas. If the comma is removed in the second example, it doesn't matter that "which" was used; the sentence retains its proper meaning for the reader.)

Test

Rule 1: Sentence's meaning is the same without the clause: use "Which"

Rule 2: Sentence's meaning changes with the clause: use "That"

Rule 3: Clause seems to naturally call for a comma: check Rule 1

Quiz

Source