不：先行詞被不確定的數量詞修飾（any, every, each, few, little, no, some, all 等），或者先行詞是不定代詞（all, something, nothing, anything等）
I am sorry fro all the things that I did to you.
I want to buy something that will make me look younger.
避：當句中已經有 who 或 which 時，爲避免重複，只能用 that
Who is the man that is giving us the class?
Which is the book that you want to buy?