一.只用that不用which的情况：
1.先行词为all、much、few、 little 、none 、something 、anything 、nothing 等不定代词时。
例：Do you have anything that is special to you?
- 先行词被all 、every 、some 、any、 each 、no 、much等修饰时。
例：I've finished reading all the books that you lent to me.
- 先行词被数词或形容词的最高级修饰时。
例：This is the first gift that I received.
- 先行词被the only、 the last、 the same 、the very 、one of等修饰时。
例：This is the very book that belongs to him.
- 先行词既有人又有物时。
例：I'll never forget the things and persons that I saw.
- 主句是以which或who引导的特殊疑问句时。
例：Which is the watch that you lost?
Who is the person that you mentioned just now?
二. 只用which不用that的情况。
- 关系代词前有介词时。
例：Those are some trees under which you can have a good rest.
- 先行词为that或those时。
例：What's that which is under the desk?
3.在非限制性定语从句中。
例：Tom came back, which made us very happy.
