Someone please tell me the difference between which and that
Let me give you two examples to explain the correct usage of "which" and "that."
1. We recently installed new siding on our house, which was flooded last month.
Use "which" when the information in your subordinate clause ("which was flooded last month") is non-essential to the meaning of the sentence. If you took away the subordinate clause, the reader would still know what house you are referring to.
2. I returned the book that I bought last night.
You should use "that" when the information directly following it is essential to understanding the sentence. Without "that I bought last night," the reader wouldn't know which book you are talking about.
The way I remember it is by thinking of "that" as a tight knot within a sentence and "which" as a looser bow.