1、引导非限制性定语从句时，用which而不用that。
2、当关系词前有介词时，用which而不用that。
3、在一个句子中有两个定语从句，其中一个定语从句的关系代词用了that, 另一个宜用which.
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4、当关系代词后面有插入语时。例如：
Here is the English grammar which, as I have told you, will help improve your English.这就是我跟你说过的会有助于你学好英语的那本英语语法书。
5、先行词为that时。例如：
The clock is that which can tell us the time.钟是报时的装置。
附：只用that不用which的情况
1、当先行词是all, little, few, much, something, everything, anything, nothing, none等不定代词时。如：
Everything that happened then was like a nightmare.当时发生的一切就像是一场噩梦。
I will tell him all that you told me at the ball.我要把你在舞会上跟我说的话全都告诉他。
2、当先行词被only, any, few, little, no, all, just, very（恰好的，表示强调）等词修饰时。例如：
The only thing that we could do was to wait.我们能做的只是等待。
That's the very thing that we can do.那正是我们能做的事。
3、当先行词是序数词、形容词最高级或先行词被序数词或形容词最高级修饰时。例如：
The first place that we visited was the Great Wall.我们参观的第一个地方是长城。
This is the best novel that I have ever read.这是我看过的最好的小说。
4、被修饰词为数词时。例如：
Yesterday I caught two fish and put them in a basin of water. Now you can see the two that are still alive. 昨天我捉到了两条鱼，把它们放在一盆水里。你可以看到那两条鱼还活着呢。
5、主句是There be结构，修饰其主语的定语从句宜用that作关系代词修饰物。例句：
There's still a room that is free.还有一个空房间。
6、先行词中同时包括人和物时。例如：
We talked about the people and the villages that we remembered.我们谈论了我们记得的人和村子。
He asked about the factories and workers that he had visited.他问起他去过的这几家工厂和工人的情况。
7、当主句中有who, which时，而定语从句中也要用到who或which时，为了避免who…who, which…which等重叠，定语从句要用that引导。例如：
Who is the man that is standing by the door? 站在门边的那个男人是谁？
Which of the two cows that you keep produces more milk? 你养的那两头奶牛中哪一个产奶多？
8、人或物在定语从句中表语时，用that引导定语从句，而that通常也可以省略。例如：
He is no longer the man that he was.他不再是过去的他。
