1、 在“介词＋关系代词”结构中，关系代词必须用which。
如： This is the reference book of which the teacher is speaking. 这就是老师正在谈及的那本参考书。
注意： 如果介词不放在修饰事物的限定性定语从句的句首， which就可换为that。
例如： This is the question that we've had so much discussion about. =This is the question about which we've had so much discussion.
这就是我们已经多次讨论过的问题。
2.、先行词为“those＋表事物的复数名词”时， 关系代词通常只用which而不用that。
如：you should grasp well those skills which may be used in the future work.
3、在非限制性定语从句中只能使用关系词which，不能使用关系词that。
例句：The tree, which is four hundred years old, is very famous here.
那棵树已经有几百年的岁月了，在这里很有名。
扩展资料
定语从句中只能用that不能用which的情况：
1、不定代词，如anything、nothing、 the one、 all、much、 few、any、little等作先行词。
例句：There is nothing that I can do.
我没有什么事情可以做。
2、先行词有the only、the very、 the same、 the last修饰。
例句：This is the very book that I want to find.
这本书是我非常想要一直在找的书。
3、先行词为序数词、数词、形容词最高级时。
例句：This is the first composition that he has written in English.
这是他第一次参加用英文写作的比赛。
4、先行词既有人，又有物时。
例句：He talked about the teachers and schools that he had visited.
他谈论起拜访过的老师和学校。
1.当非限制性定语从句中的先行词指物时，只能用which。如：
The pencil-case ， which I bought last week ，is missing.
那个铅笔盒，我上周买的，现在不见了。
2.关系代词前面有介词时，只能用which。如：
The tree under which we used to take a rest has been cut down.
我们过去常在下面休息的那棵树已砍掉了 。
3.先行词为代词that或that所修饰时，只能用which 。如：
I don't take that which is too expensive.我不要太贵的那一个
1.当非限制性定语从句中的先行词指物时，只能用which。如：
The pencil-case ， which I bought last week ，is missing.
那个铅笔盒，我上周买的，现在不见了。
2.关系代词前面有介词时，只能用which。如：