只能用which，不用that的情况：
1）先行词为that，those时，用which，而不用that.
What’s that which is under the desk？在桌子底下的那些东西是什么？
2）关系代词前有介词时，一般用which，而不用that.
This is the room in which he lives. 这是他居住的房间。
3）引导非限制性定语从句，用which，而不用that.
Tom came back，which made us happy. 汤姆回来了，这使我们很高兴。
