定語從句只能用which的情況：1.在介詞后面的關系代詞用which而不能用that。即“介詞+which（代物）”；2.在非限定性定語從句中不能用that等。
定語從句中只能用which，不能用that的幾種情況
1.在介詞后面的關系代詞用which而不能用that。即“介詞+which（代物）”例如：
The picture for which he paid $1,000 was once owned by a duke.
他花了1000美元買下的畫曾為一名公爵所擁有。
2.在非限定性定語從句中不能用that。例如：
Crusoe's dog, which was very old now, became ill and died.
克魯索的狗，現在已經很老了，病死了。
3.有兩個定語從句，其中一個關系代詞已有which，另一個可以用that。相反，如果其中一個為that，另一個可以用which。
Edison built up a factory which produced things that had never been seen before.
愛迪生辦了一個工廠，生產過去從未見過的東西。