定语从句只能用which的情况：1.在介词后面的关系代词用which而不能用that。即“介词+which（代物）”；2.在非限定性定语从句中不能用that等。
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。
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，另一个可以用that。相反，如果其中一个为that，另一个可以用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。
