which可以引导定语从句、修饰表示事物的先行词.在定语从句中充当主语、宾语或者表语.
例如：
He was reading a book,which was about war.他正在读一本关于战争的书.关系代词which在修饰book的定语从句中充当主语.
He was reading a book,which he had bought from London.他正在读一本书,这本书是从伦敦买回来的.关系代词which在修饰book的定语从句中充当宾语.
which引导非限定性定语从句时有一个特殊用法、即它所引导的非限定性定语从句修饰主句中更多的部分甚至整个主句、这时被定语从句所修饰的“先行词”要作广义理解、which仍在定语从句中作主语、宾语或表语.
例如：
He set free the birds happily,which was a celebration for his success.他开心地把鸟放了,这是对他成功的一种庆祝.
which也可引导名词性从句（宾语/定语/表语/同位语从句）.
表语从句：My question is which student is the best at English.我的问题是哪个学生最擅长英语.
宾语从句：I don't know which bike belongs to my father.
主语从句：which boy she likes best is still a mystery.
同位语从句：I have no idea which picture is the most beautiful of all.