that什么时候不能用要用which
1、只用which不用that的情况。
2、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。
3、当关系词前有介词时。例如：this is the room in which chairman mao once lived.这就是毛曾经住过的地方。
4、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。
5、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。
6、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。
