在一个复合句中，跟在主句后修饰某一名词或代词（统称为先行词）的从句叫做定语从句(The Attributive Clause)。
The woman who is standing by the classroom is our Chinese teacher.
This is the room where Bin Ladin once lived.
The girl who plays the piano very well is his girlfriend.
She is no longer the woman who she used to be.
I can't remember the man who I gave the key to.
Where is the student whom Professor Nirth praised at the meeting.
The girl with whom he was traveling is his girlfriend.
She is looking after the little child whose parents have gone to Singapore.
He lived in the room whose windows face south.
The man that is speaking at the meeting is a famous scientist.
Is this the teacher that you talked about yesterday.
She is no longer the girl that she was in her childhood.
The train that has just left is for Beijing.
Is this the photo that you took last summer?
I wish my home town was not a polluted place that you think it to be.
The building which stands near the train station is a company.
The computer which she wanted to buy was sold out.
She was fond of dancing,which her husband never was.
at,on,in,during + which)：代替时间名词，做时间状语
I shall never forget the day when(on which) we moved into our new flat.
in,at + which)：代替地点名词，做时间状语
This is the place where(at which) John parks his car.
She didn't know the reason why(for which) she was dismissed.
Here are such questions as are often asked by the universtiy students.
This is the same T-shirt as I have bought.
She has never seen such a handsome boy as I am.
She'll do the experiment the same way as I do.
I came the same day as he left.
She studies in the same college as he does.
She didn't believe such reason as he did so.
Here is so difficult a question as nobody can answer.
as we all know,
as is known to all,
as it is,
as is said above,
as already mentioned above,
as is usual,
as is often the case,
as is reported in the newspaper等句式中。
As is known to China,Qu Yuan is a great Chinese poet.
He forgot to bring his textbook with him,as was often the case.
She is absorbed in work,as she often was.
As we all know,the moon is round.