http://www.engvid.com/ Learn when to use "that" and "which," and listen to a review of the two types of relative clauses. Commas are important and can change the entire meaning of a sentence! Make sure to also watch my lesson on relative clauses at http://www.engvid.com/writing-relativ... and to take the quiz on this lesson at http://www.engvid.com/that-which/#quiz