关系词通常有下列三个作用:1)引导定语从句;2)代替先行词;3)在定语从句中担当一个成分。

关系代词有that,which,who,whom,whose,as等;

一:关系代词的用法 关系代词 指代对象及在从句中所作的成分
使用要点
that
即指人又指物;作主语或宾语(作宾语时可省略)
只用于限制性定语从句中,介词后面不能跟that。 当代替物时,可以与which通用。
Yesterday Emily was wearing the new dress that I gave her.
昨天艾米丽穿了我送给她的新衣服。
I have a friend that likes listening to classical music我有个朋友喜欢听古典音乐。 

which
指物;作主语或宾语(作宾语时可省略)
在非限定性定语从句中,只宜用which,不能用that.在限定性定语从句中,可用在介词之后。
My parents live in a house which is more than 100 years old
我的父母住在一座具有100多年历史的房子里 

who,whom
指人;作主语或宾语(whom常可省略) 先行词必须为人。Who可代替在从句中担任宾语的whom,若前有介词,须用whom
A couch potato is a person who spends a lot of time watching TV. “A couch potato”指那种成天沉湎于电视中的人。

The student(whom)I taught two years ago has joined the army.
我两年前教的那个学生已经参军了。 

whose 既指人又指物;作定语 是代词的所有格, 代物时,相当于of which。
The room whose window faces south is mine. 那个窗户朝南的房间是我的。
注意:whose指物时,常用of which结构来代替。如:
The classroom whose door is broken will soon be repaired.
=The classroom the door of which is broken will soon be repaired.