1． 关系代词有：that, which, who, whom,在定语从句中作主语、宾语和表语，whose
作定语。代人的有：who, whom, whose, that；代物的有：that, which, whose.例如：The man who helped you is Mr. White. (who代人，在定语从句中作主语)
That is the person(whom/who/that) you want to see.
(whom, who, that代人，在定语从句中作宾语，可省略)
A dictionary is a useful book which(＝that) tells us the meaning of words
（注意：whose在从句中做定语指某人的也可以用…of whom代替，指物时也可以用…of which 代替）
The professor whose daughter teaches you English is Dr. Williams（=The professor, the daughter of whom teaches you English, is Dr. Williams. ）
Have you seen the film “Tatanic”, whose leading actor is word famous?
( = Have you seen the film “Tatanic”, the leading actor of which is word famous?)
The house where they live is not very large.
This is the reason why he did not come to the meeting.
We’ll never forget the day when the People’s Republic of China was founded.
I know the reason why she studies so well .
词；非限制性定语从句常用逗号和主句隔开，可以修饰主句中的某一个词，也可修饰整个句子。非限制性定语从句不能用that引导，引导非限制性定语从句的关系词不能省略。例：Li Ping’s father, who works in a factory, is an engineer.
Dr. Li, whom I know very well, will come here tomorrow.李大夫明天将来这儿，我跟他很熟悉。He studies hard at school when he was young, which leads to his success in his later life . John was admitted into the college, as we had expected.约翰被大学录取了，这是我们期待的
2. which 和as在做非限制性定语从句关联词时可以无明确先行词，指前句整个的意思。但通常As 可以放在整句的句首（也可以在逗号之后），而which只能在逗号之后。as 本身含有“正如”之意， She is honest, as (which) I have told you.
He is honest, which (as) we can see. 他很诚实， 这一点我们看得出来。
（但前后两句是因果关系时which不可换为as而且在as is known/believed, as we all know/believe这类结构中as不可换为which)
He tore up my photo, which made me very angry. (which指代主句内容，因前后两句是因果关系，此时which不可换为as。)
Tom was late for school again and again, which made his teacher very angry.
He is good at physics, as is known to us all.（＝As is known to us all, he is good at physics.）
As you know, she has read a lot of novels.
A. I know a place where we can have a picnic.
I know a place which / that is famous for its beautiful scenery. (从句中缺少主语，不可用where)
B. I will never forget the days when we spent our holidays together.
I will never forget the days that / which we spent together. (从句中spent后缺宾语，不可用when)
C. This is the reason why he was dismissed.
This is the reason that / which he explained to me for his not attending the meeting. 这就是他向我解释的他没有参加会议的原因。(从句中explained后缺少宾语，不可用why )
1） 当先行词既包括人又包括物时：He talked about the men and the books that attracted
They talked for about an hour about things and persons that they remember in the school. 他们谈起他们所记得起的学校里的人和事，谈了大约有半个小时
2） 指物的先行词被any, every, only, very, all, no，the only 、the very、the last 等修饰时：
That’s the only watch that I like most.
The first thing that we should do is to work out a plan.
I t almost seemed as if the good man were trying to teach us all that he knew at this lesson.
This is the very dictionary that I want to find.
This is the second card that he gave me.
This is the best film that I have seen .
4） 先行词是指物的不定代词all, little, few, much,及everything, anything, nothing等时：There is still much that can be done about it.
Have you got everything that you need?
Is there anything (that) I can do for you?有什么我可以帮助做的事吗？I’ve brought everything (that )you need.我把你需要的东西都拿来了
5） 先行词是who或主句以who、which开头的特殊疑问句时，定语从句要用that ：Who that have seen him does not like him?
Who is the man that is talking with Tom ?正在和汤姆谈话的人是谁？Which of the books that you bought is easy to read?你买的那些书中哪一本容易读
Last night I saw a very good film, which was about the Long March（长征） .
This is the man from whom I learnt the news
I have found that which I was looking for.
Can you lend me the magazine about which you talked yesterday? (talk about……)
This is the dress for which he paid 10 pounds. ( pay + money + for sth.)
This is the child of whom we are proud. ( be proud of……)
She wants find the book in which she is interested . ( be interested in…..)
2． 介词常受先行词的制约（即介词和先行词的搭配），例如：He built a telescope through which he could study the skies.(through which =through the telescope)
The factory in which my father works makes telephones. (in which = in the factory)
The gun with which he was shot was never found. (with which = with the gun)
例This is the watch which you’re looking for.
He is a kind of man whom you can safely depend on.？
1. situation, case, scene, position和point（表示抽象的地点）后常用where或in which引导定语从句：
Can you imagine a situation where（in which） you can use the word?
He is now in a position where he can earn a lot of money. 他现在处在一个很挣钱的岗位
At last the couple came to the point where they would separate.
2. way (方式，方法)后常用in which或that或how引导定语从句，也可省略：Do you know the way (in which/that/how) he worked out the problem?
3. “the same＋名词”， “so + adj +名词”, “such＋名词”，“as＋名词”后通常用as引导定语从句。
I have the same dictionary as you (have).
Such a book as you bought is not worth buying.
She is wearing the same coat as you saw yesterday.
She is wearing the same coat that her father bought yesterday.
From the years when (in which) he was going to primary school in the country he had known what he wanted to be when he grew up. (Which = the years when = in the years = in which )
Great changes are taking place in the city where (in which) they live.
(which = the city where = in the city = in which)
The reason why (for which) he refused the invitation is quite clear.
(which = the reason why = for the reason = for which)
This is the room where (in which) he did his homework..
(which = the room where = in the room = in which)
The plane that has just taken off is for Paris. （定语从句）
The fact that he has already died is quite clear. （同位语从句）
2．定语从句由关系代词或关系副词引导，关系词在从句中担当相应的句子成分，关系代词在从句中作宾语时经常可省略。同位语从句主要由连词that 引导，在从句中一般不担当成分；有时也由where, when, how, who, whether, what 等连词引导，这些连词则在从句中担当成分。
The news that he told me is true. （定语从句）
The news that he has just died is true. （同位语从句）
The problem that we are facing now is how we can collect so much money. (定语从句)
The problem how we can collect so much money is difficult to solve. （同位语从句）
The question that he raised puzzled all of us. （定语从句）
The question whether he is sure to win the game is hard to answer. （同位语从句）
A. The idea that he we could ask the teacher for advice is wonderful. （同位语从句）
(The idea was that we could ask the teacher for advice.)
B. The fact that the earth moves around the sun is known to all. （同位语从句）
(The fact is that the earth moves around the sun.)
C. Please pay attention to the problem how we can protect the wild animals. （同位语从句）
(The problem is how we can protect the wild animals).