2) 必在从句中作某个句子成份（可以做主语，宾语，表语，定语，状语） 常用的关系代词: that、 which、 who、whom、whose,当关系代词在定语从句中做宾语时，关系代词可以省略。
常用的关系副词（在从句中只作状语）: when（时间状语）、why（原因状语）、 where（地点状语）
可以修饰人的关系代词：that, who, whom, whose
可以修饰事的关系代词：that, which, as, whose,
The student who answered the question was John.
I know the reason why he was so angry.
The boy (whom) you are talking to is my brother.
I'd like a room whose window faces the sea.
●that: 可指人或物；在定语从句中作主语，宾语，表语。(指人时，相当于who或 whom；指物时，相当于which)(一般不用于非限制性定语从句; 不可置于介词后作宾语) 如：
1. A letter that/which is written in pencil is difficult to read. (主语)
2. Do you know the gentleman that/who spoke just now?
3. You can take anything ( that) you like. （宾语）
4. What is the question (that/which) they are talking about?
5. Here is the man ( who/whom/that ) you want to see.
6. She's no longer the girl ( that) she used to be before.（表语）
7. Our hometown is no longer the one (that) it used to be.
（= Our hometown is not the same as it used to be.
= Our hometown is different from what it used to be。
= Our hometown is not what it used to be.）
1. The book which/that was on the desk was bought by my father.（主语）
2. The book (which/that) I bought yesterday is very interesting.（宾语） ●who, whom, whose:
who: 主格, 在从句中作主语，只可指人
whom: 宾格，在从句中只能作宾语; 只可指人
whose + 名词 =the + n. + of which (某物的)
the + n. + of whom (某人的)
I like the students who/that work hard. （主语）
The boy (that/ who / whom ) we saw yesterday is Tom.
I know the girl whose father is a teacher.
(whose father = the father of whom)
A child whose parents are dead is called an orphan.（指人）
(whose parents = the parents of whom)
I'd like a room whose window faces south. （指物）
(whose window =the window of which)
关系代词作介词宾语 (在定语从句中，介词提前时，介词后：表人用 whom；表物用 which)
关系代词在定语从句中用作介词宾语时, 介词可放于从句之首, 也可放于从句之末. 但以放于句首较为正式.（介词前置，必须注意不影响动词词组的含义。关系代词who 和 that 用作介词宾语时, 介词必须放在句末.）
The man (whom / who / that ) my mother is talking with is Mr Black.
= The man with whom my mother is talking is Mr Black.
This is the book for which you asked.
=This is the book (that/which) you asked for.
Do you know the person with whom I shook hands？
= Do you know the person (whom/who/that) I shook hands with?
The library from which we can borrow books is very big.
This is the book about which I told you just now.
●as 的用法:(as 引导定语从句, 在定语从句中作主语、宾语、表语)
①如为限制性的，多用于the same …as ; the same as(与……相同)；such …as …（如此，这样）; as many/much as（和……一样多）；so/as …as（与……一样）等结构中。如：
※I have the same book as you (have). 我有一本和你的一样的书。
Her attitude to him was quite the same as it had always been. (关系代词as和指示代词same连用, 在从句中用作表语, 先行词是same.)
.---Why didn't you mention that in face of the police just now?
--- I thought it was such a minor detail as was hardly worth mentioning.
We will only discuss such problems as have something to do with our own interests.
Don't do such things as you are not sure about.
There is no such place as you dream of in all this world.
比较：I live in the same house that he used to live in.
I'm wearing the same shirt as you wore yesterday.
比较：Here is so big a stone as no one can lift. （定语从句）
Here is so big a stone that no one can lift it.（结果状语从句）
②如为非限制性的，as还可以单独引导一个定语从句，代替整个主句，这种定语从句可置于句首，句中或句尾，译为"正如，这一点"。（动词常为know, see, expect, point out, etc.）
As we all know, smoking is harmful to one's health . (as 作宾语)
=As is known to all, smoking is harmful to one's health . （as 作主语） 众所周知：As we all know,
It’s well-know (to us all) that…
=It's known to all that smoking is harmful to one's health .
=Smoking is harmful to one's health , as we all know .（as 作宾语）
=Smoking, as we all know, is harmful to one' health.
He was a foreigner, as I knew from his accent. (宾语, 先行词是前面整个句子)
Ⅲ. 关系副词(when, where, why)引导的定语从句,它们后面的句子完整，只缺状语，它们都可以等于 介词+ which , 其中 why 只等于 for which.
其先行词是表时间的名词（如：time, day, week, tear, month, etc.）
He came last night when I was out.
We will put off the picnic until next week, when the weather would be better. 注意：先行词为"时间名词"，可用when引导定语从句，when在定语从句中作状语；还可以用which或that 引导，which或that在从句中作主语或宾语。 比较:
I still remember the day when /on which my brother joined the army.（作状语） I still remember the days which/that we spent together. （作谓语 spent 的宾语） Next winter which/that you'll spend in Harbin, will be exciting.（宾语） I shall never forget the day when Shen Zhou Ⅴ was launched.
There are occasions when joking is not permissible.
其先行词是表示地点的名词，如：place, school, factory, room, etc.
This is the place where I was born.
I live in the room where /in which he used to live.
比较: ※This is the factory where /in which he worked last year. （作状语） This is the park which/that they visited last year. （作宾语）
先行词为reason 时，可用for which指代；当关系词在从句中作主语或宾语时，则用which或that 引导。如：
The reason why + 定语从句 is that + 表语从句 （……的原因是……）
The reason why / for which / (that) he didn't come to school was that he was ill. I don't believe the reason (that/which) he gave me. （作宾语）
He asked me the reason that can explain my success. （作主语）
He was late .That's because he got up late.
(That’s because + 原因) 那是因为……
He got up late. That's why he was late.(表语从句)
（That’s why + 结果） 那是……的原因
(the reason) why/for which he was late. (定语从句)
当先行词为way时，定语从句常用that, in which,或省略引导词
This is the way (that) /（in which）I do such things.
比较: Please do the experiment in the way （that/which）I have shown you. Ⅳ. 限制性定语从句与非限制性定语从句的区别：
This is the book I like best. 这就是我最喜欢的那本书。
Beijing, （which has been China's capital for more than 800 years ,） is rich in cultural and historic relics. 北京是中国八百年之久的古都，它有着丰富的文化和历史遗产。
比较: He has a sister, who is a musician.
He has a sister who is a musician.
引导非限制性定语从句的关系代词，指人时用who, whom, whose , 指物时用which , whose; 关系副词when，where, why, etc. later life.（which 代替整个前面整个主句）
2. Tom's father, who arrived just now, is a famous scientist.
3. They set up a separate state of their own, where they would be free to keep Negroes as slaves.
4. He was proud, which his brother never was.
●that & which:
在定语从句中，which 和that 在指代事物时，一般可以互换使用，但并非在任何情况下都是这样，这里介绍宜用that, 而不宜用which 的情况.
①先行词为much, little, none, all, few, every(thing), any(thing), no(thing), the one等不定代词或受其修饰时;
1.We should do all that is useful to the people .
2.There's nothing that can be said about it .
3.Do you mean the one that was bought yesterday?
②先行词被the only, the very修饰时;
1.The only thing that we could do was to wait.
2.That's the very word that is wrongly used.
比较 *This is one of the best novels that were published last year.
*This is the only one of the best novels that was published last year.
③先行词是序数词时或被序数词修饰时,包括the last, the next;
1.When we talk about Wuxi, the first that comes into mind is Tai Lake.
2.This is the third film that has been shown in our school this term.
1.This is the best that can be done now.
2.The most important thing that should be done right now is how to stop him from going on.
⑤先行词既有人又有物, 只用that. 如：
1.The writer and his novel that you have just talked about is really well known .
2.The rider and his bike that had run over an old woman were held up by the police.
1.Yesterday I caught two fish and put them in a basin of water .Now you can see the two that are still alive .
Edison built up a factory which produced things that had never been seen before. ⑧主句是以who或which开头的疑问句时，关系代词宜用that，以避免重复。
1. Which is the book that you like best?
2. Who is the man that is standing at the gate?
⑨主句是There be 结构，修饰其主句的定语从句宜用that 作关系代词. 如：
1. There is still a seat in the corner that is still free.
1. He is not the young man that he was 30 years ago.
2. My home village is no longer the place ( that ) it used to be .
1.A zoo is a park in which many kinds of animals are kept for exhibition.
2.Is this the room in which Mr. White lives?
1.Crusoe's dog, which was are now very old, became ill and died .
2.More and more people are beginning to learn English, which is becoming very popular in our country. (which指代主句)
③在一个句子中有两个定语从句，其中一个定语从句的关系代词用了that, 另一个宜用which .
1. Let me show you the novel，that I borrowed from the library which was newly open to us.
2. At the station I bought some magazines that might help me to pass the time on the train and which I could pass on to others when I finished them.
1. Here's the English grammar which, as I have told you, will help improve your English.
⑤先行词本身是that, 宜用which .
What's that which she is looking at?
A shop should keep a stock of those goods which sell best.
●as & which:
as & which 引导非限制性定语从句的区别：
Which（放句中） 引导的定语从句只置于所限制的句子后；as 位置较灵活，也就是说as可置于所限制的句子前面；插在句子中或放在句子后。如：
1. He was late again, which made his teacher very angry.
2. Jack, as you know, is an honest man. 或Jack is an honest man , as you know. 或As you know, Jack is an honest man.
which引导非限制性定语从句时，其先行词可以是一个词，一个短语或一个句子。 He was proud, which his brother never was. （先行词是一个词）
She was very patient towards the children, which her husband seldom was. He was proud, which I dislike very much.（先行词是一个句子）
He is an honest man, as is known to all.
He was a foreigner, as I know from his accent.
as we all know；as you know; as is known to all; as you see; as we can see; as has been expected; as we have imagined.
·The same…as 与the same…that 的区别
The same…as 指的是同一类事物, 例如
She bought the same shirt as I wear.
The same…that 指的是同一个事物，例如
The police found the same car that I lost last week.