"Which" and "that" are relative pronouns used to introduce dependent clauses, but their usage depends on the type of clause. "That" is used in restrictive clauses, which are essential to the meaning of a sentence. For example, "The book that I borrowed was excellent." The clause "that I borrowed" specifies which book and is essential to understanding the sentence. "Which," on the other hand, introduces non-restrictive clauses, which add extra information but aren't essential to the sentence's meaning. For example, "The book, which I borrowed from the library, was excellent." The clause "which I borrowed from the library" adds extra information, but the sentence would be clear without it. Remember, non-restrictive clauses are usually set off by commas. Learn More: Which vs That