关系代词which一般指物，在定语从句中作主语或宾语，既可以引导限制性定语从句，又可引导非限制性定语从句。另外“介词+which”具有替代作用，十分常见。
1.引导限制性定语从句
（1）A shop should keep a stock of those goods which sell best.
商店应存有最畅销的货物。
（2）This is the family which is planning to move to the party.
这是要搬进城里的一家。
2.引导非限制性定语从句
（1）用来指代一个句子
Internet is so interesting,which makes all possible happen.
因特网如此有趣，它可以使任何事都有可能发生。
（2）用来指代句子的一部分
When deep in thought,which he often was,he would forget all around him.
他常常陷入沉思，这时他就会忘掉周围的一切。
（3）如果要引导两个非限制性定语从句，第二个which前要加and
He bought a book,which was written by LuXun,and which he decided to give to his friend.
他买了本鲁迅写的书，他决定送给朋友。
3.名词+of+which（=of which+名词=whose+名词）通常放在先行词的后面。
I’d like a room the window of which looks out over the sea.
I’d like a room of which the window looks out over the sea.
I’d like a room whose window looks out over the sea.
我要一间窗户面临大海的房间。
4.介词+which的替代作用
（1）作时间状语替代when
There used to be a time at which/during which（=when）the Chinese people struggled for freedom.
中国人曾有一段为自由而斗争的日子。
（2）作地点状语替代where
This is the office in which（=where）I used to work.
这是我过去工作过的办公室。
（3）作原因状语替代why
I’d like you to explain the reason for which（=why）you were absent.
我想让你解释一下缺席的原因。
（4）作方式状语替代that或省略
There are many ways in which（=that/省略）we can solve the problem.
解决这个问题有很多方法。