1. 引导非限制性定语从句时。例如：Bamboo is hollow, which makes it light.竹子是空心的，这使得他很轻。
2、当关系词前有介词时。例如：This is the room in which Chairman Mao once lived.这就是毛主席曾经住过的地方。
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.例如：Let me show you the novel that I borrowed from the library which was newly open to us.我来给你看我从新开的图书馆里借来的那本小说。
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。
答:在引导非限制性定语从句时，只能用which。
例句:But that which has no time cannot be measured by words.
但是没有时间的那个东西是无法用语言衡量的。
