在句中做定语，修饰一个名词或代词，被修饰的名词,词组或代词即先行词。定语从句通常出现在先行词之后，由关系词(关系代词或关系副词)引出。
关系代词有：who,whom,whose,that,which等。
I like the books that are written by Mr.Green.
A.关系词：
关系词通常有三个作用：引导定语从句，指代先行词，在定语从句中充当句子成分。
Anyone who breaks the law will surely be punished.
1.先行词指人时，定语从句的引导词：
(2)who和whom都可以作定语从句宾语，但从句中的介词提前时，不能用who.
He is the man who/whom I talked to you about.
=He is the man about whom I talked to you.
(3)先行词是人时，只用who,不用或少用that的情况：
a.当先行词是one,ones,anyone时，引导定语从句的关系词一般只用who,不用that如果先行词是someone,也可用that
He is not one who is easily frightened.
I think Joe is the one who borrowed my peo.
The ones who tell lies won’t gain others’trust.
Anyone who saw the accident should phone the police.
Someone who/that claims to be your uncle wants to see you.
b.先行词是those时，关系代词一般用who不用that
Those who don’t wish to go need not go.
c.There be句型中，修饰人的定语从句的关系代词一般只用who来引导
There is a certain old man in the village who none of us likes.
d.若一个句子中有两个定语从句，第一个已经用that,则第二个一般用who
The man that is talking with our teacher is the professor who gave us a report yesterday.
e.当定语从句与先行词较远时，引导定语从句的关系代词一般只用who
I came across my first teacher in the store yesterday who was still beautiful.
He has a son,who is a doctor.
(4)当先行词是人，只用that不用who的情况：
a.以who开头的特殊疑问句，定语从句的先行词如果是人，关系代词只用that不用who
Who is the girl that said hello to you just now?
b.当先行词既有人也有物时，定语从句只用that来引导
They ofter talk about the person and things that they remember.
c.当关系代词在定语从句中作表语时，只用that来引导
He’s changed.He is not the man that he was.
d.当先行词被形容词的最高级修饰时，只用that.
This is the most beautiful lady that I have ever met.
e.当先行词是other时，定语从句只用that引导。
You can ask the old man or any other that was there at the time.
