Use "that" whenever the identity "that" is replacing is known to the speaker. Examples: I will read the book that is written by my favorite author. (not: "which is written..") All that I can see from here is beautiful. She didn't even apologize, and that is why I am angry. (not: "apologize, which is why...") Use "which" whenever the speaker does not know what "which" is. Examples: Which route will we take to get there? I do not know which [one] is the better car. Some people adhere to a rule that says in effect use "that" for restrictive relative clauses and "which" for non-restrictive relative clauses. A good discussion of this topic can be found [url=" http://www.bartleby.com/64/C001/062.html "]here[/url]. My own personal belief is that this rule has been relaxed over time. Now you can use "that" and "which" much more interchangably with restrictive and non-restrictive relative clauses. But some still rigourously adhere to this rule. The relative pronoun "which can introduce both restrictive and non restrictive relative clauses, as you will read in any fairly good grammar. "The house which I bought last year has three bedrooms." (restrictive) "The house, which I bought last year, has three bedrooms." (non-restrictive) On the other hand, you will probably not find "that" as the introductory pronoun of a non-restrictive relative clause in a text written by anyone who has some knowledge of grammar. "The house (that) I bought last year has three bedrooms." "The house, that I bought last year, has three