定语从句中只能用which，不能用that的几种情况：
1. 在介词后面的关系代词用which而不能用that。即“介词+which（代物）” 例如：
The picture for which he paid $1,000 was once owned by a duke.
他花了1000美元买下的画曾为一名公爵所拥有。
The building in which Han Mei studied was very old.
韩梅在里边学习的那幢大楼很旧。
2. 在非限定性定语从句中不能用that。例如：
Crusoe's dog, which was very old now, became ill and died.
克鲁索的狗，现在已经很老了，病死了。
3. 有两个定语从句，其中一个关系代词已有which，另一个宜用that。相反，如果其中一个为that，另一个宜用which。
Edison built up a factory which produced things that had never been seen before.
爱迪生办了一个工厂，生产过去从未见过的东西。
Let me show you the novel that I borrowed from the library which was newly open to us.
让我给你看看我从新开放的图书馆借来的小说。
