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.
The building in which Han Mei studied was very old.
Crusoe's dog, which was very old now, became ill and died.
Edison built up a factory which produced things that had never been seen before.
Let me show you the novel that I borrowed from the library which was newly open to us.