1. Why is there a comma before the word "which" but not before the word "that" ( both can have the same/simular meaning)?
2. When exactly is a comma used before the word "which"?
A. "That" usually introduces an essential clause, while "which" normally introduces a nonessential clause. Since a nonessential clause can be taken out of the sentence without significantly altering the mean, the clause is separated by commas.Here are some examples:
The plan that he had in mind was sure to work. (Identifies a specific plan. If you take "that he had in mind" out of the sentence, the meaning is changed.)
The new policy, which I didn't approve of, is sure to be a disaster. ("Which I didn't approve of" in this sentence is just added information. You can take it out of the sentence without altering the basic meaning of the sentence.)