1、只用which不用that的情況。

2、引導非限制性定語從句時。例如：Bamboo is hollow, which makes it light.竹子是空心的，這使得他很輕。

3、當關系詞前有介詞時。例如：This is the room in which Chairman Mao once lived.這就是毛主席曾經住過的地方。

4、在一個句子中有兩個定語從句，其中一個定語從句的關系代詞用了that，另一個宜用which.例如：Let me show you the novel that I borrowed from the library which was newly open to us.我來給你看我從新開的圖書館里借來的那本小說。

5、當關系代詞后面有插入語時。例如：Here is the English grammar which, as I have told you, will help improve your English.這就是我跟你說過的會有助于你學好英語的那本英語語法書。

6、先行詞為that時。例如：The clock is that which can tell us the time.鐘是報時的裝置。