不：先行词被不确定的数量词修饰（any, every, each, few, little, no, some, all 等），或者先行词是不定代词（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：当句中已经有 who 或 which 时，为避免重复，只能用 that
Who is the man that is giving us the class?
Which is the book that you want to buy?