2. 把名词给限定了，tall man(很多人中，高的那个），从多个名词中确定一个。
3. 可以放在名词前面或后面（a beautiful girl）[名词之前，限定girl], a girl who is beautiful.[名词之后，通常为定语]
The big woman served me.
The woman who has a handbag served me.
人 + who/that + 动词...
物 + which/that + 动词...
The people are very friendly.[句子主干]
The people who work in the office are very friendly.[定语从句只跟它修饰的名称有关]
The woman is a doctor.
The woman who lives downstairs is a doctor.
Do you know the woman?
Do you know the woman who wants to buy the car?
I want to marry a man who loves me.
I want to marry a man who I love.
The dinner was delicious.
The dinner my mother cooked was delicious.
The fish was dead.
The fish (that) my wife bought was dead.
I found the book I wanted.
I met a woman who can speak 6 languages.
What's the name of the woman who lives next door?
What's the name of the river which flows through the town?
Where is the picture which was hanging on the wall?
Do you know anybody who wants to buy a car?
You always ask questions that are difficult to answer.
A thief is a person who steals things.
A dentist is a person who looks after your teeth.
A butcher is a person who sells meat.
A fool is a person who is very stupid.
A musician is a person who plays a musical instrument.
A genius is a person who is very intelligent.
Barbara works for a company that makes furniture.
The book is about a girl who runs away from home.
What happened to the pictures that were on the well?
A mystery is something that can not be explained.
The police have caught the men who stole my car.
A dictionary is a book that gives you the meaning of words.
all, little, few, much, none, everything, anything
Is there anything that I can do for you?
Tom told his mother all that had happened.
Is there any news that is about the entrance examination?
I'd like to read all the books that were written by him.
This is the best book that I have read.
It's the most interesting film that I have ever seen.
4. 名词有序数词(first, second, next, last...)修饰
This is the last thing that I want to do.
Are there any rooms?
The rooms' windows face the sea.
Are there any rooms whose windows face the sea?
We'll go to the town.
I have heard of the name of the town.
We'll go to the town whose name I have heard of.
This is the place which I lived in.
This is the place in which I lived.
This is the place where I lived.
2. 介词在定语从句句尾太孤独，挪到which 前，更正式
You are the guy we are trying to get away from.
You are the guy from whom we are trying to get away.
I'm living in a huge house in which I can even I can play golf.
The driver was the man from whose room she had stolen the maps.
表示抽象地点的词（ point, case, position, condition, situation), 常用where引导
where = 介词 + which
You have reached a point where medicine can not help.
= You have reached a point at which medicine can not help.
It's put me in a position where I can't afford to take the job.
= It's put me in a position in which I can't afford to take the job.
When = 介词 + which
We have all experienced days when everything goes wrong.
We have all experienced days on which everything goes wrong.