引导定语从句的引导词（也叫关联词）可以是关系代词（that, which, who, whom, whose）或者关系副词（when, where, why）。
That
既可用来指代人也可用来指代事物，在从句中充当主语或宾语。
Who’s the boy that is reading a book?
The man that you talked to is my uncle.
The train that has just left is for Shanghai.
Who和Whom
who指人，在从句中做主语
The boys who are playing football are from Class One.
Yesterday I helped an old man who lost his way.
whom指人，在定语从句中充当宾语，常可省略。
Mr. Liu is the person (whom) you talked about on the bus.
Mr. Ling is just the boy whom I want to see.
Whose
whose通常指人,也可指物,在定语从句中做定语
He has a friend whose father is a doctor.
I once lived in a house whose roof has fallen in.
whose指物时,常用以下结构来代替
The classroom whose door is broken will soon be repaired.
The classroom the door of which is broken will soon be repaired.
Do you like the book whose cover is yellow?
When
when表示时间，在定语从句中作时间状语。
Could you still remember the time when the train left?
I’ll never forget the date when I joined the League.
Where
where表示地点，在从句中作地点状语。
Is this the shoe factory where your father works?
Why
why指原因,在定语从句中做原因状语
Please tell me the reason why you missed the plane.
I don’t know the reason why he looks unhappy today.