一、定语从句的种类：定语从句可以分为限定性定语从句和非限定性定语从句。
1. 限定性定语从句：它是先行词在意义上不可缺少的定语，如果去掉，主句的意思就不完整或失去意义。这种从句与主句的关系
十分密切，书写时不可用逗号分开。如果关系代词在限定性定语从句中作宾语，关系代词通常可以省略，如：
Do you know the girl who just came in?
Shanghai is a city（that）I’ve always wanted to visit.
Her sister married a man (who/whom/that) she met on a plane.
The time when I first met Mr. White was a very difficult period of my life.
2. 非限定性定语从句：它只是对先行词作附加补充说明，如果去掉，主句的意思仍然清楚完整。这种从句与主句的关系不是很密切，书写时往往用逗号分开。非限定性定语从句不用that引导，而且不可以省略关系词。
The old woman, who lives on her own, has a cat for company.
Lori is going to marry Mark, whom she does not love.
Beijing, which is the capital city of China, is a very beautiful city.
二、关系词分类：关系代词和关系副词
1. 根据先行词选择的关系词，分为关系代词和关系副词，具体如下：
（1）关系代词
类别 | 关系代词 | ||||
指人 | who 主、宾 | whom 宾 | that 主、宾、表 | whose 主、宾 | as 主、宾、表 |
指物 | which 主、宾 | ||||
（2）关系副词：when, where, why
（3）关系词的三重作用：连接主句与从句，指代先行词和在从句中作句子成分
★以上关系词，that不能用于非限定性定语从句，其他均可用于限定性定语从句和非限定性定语从句中。
2. 关系副词 where, when, why
The school where I learned judo was very large.
I remember the day when our school football team was formed.
I don’t know the reason why she got so angry this morning.
三、用法
1. 关系代词的选用取决于主句中的先行词以及关系代词在从句中作何种成分。
1) 关系代词who, whom的用法
(1) who的先行词必须是人，在从句中担任主语，往往也可以代替在从句中担任宾格的whom,但它的前面不能有介词，如果带介词则必须用宾格whom，即“介词 whom”.
Jonny is a person who always has novel ideas.
The girl who got the first prize in the contest is from Zhejiang.
The person who/whom you just talked to is Mr. John.
The person to whom you just talked is Mr. John. (介词to提到定语从句前，只能用whom.)
We’ll go to the hospital to see the patients, most of whom are children.
(2) 在定语从句中，指人时who, that可以通用，但在下列情况下用who, 不用that.
A) 先行词是one, ones, anyone, no one, all, nobody, anybody等词时，用who.
One who has nothing to fear for oneself dares to tell the truth.
The ones who flatter me don’t please me.
Anyone who laughs last laughs best.
Anyone who fails to finish the task given should be criticized.
B) 先行词为those时，用who.
Those who want to go to the Great Wall sign up here.
No words are strong enough to express our thanks to those who worked hard to rescue survivors in the earthquake.
C) 当先行词有较长的后置定语时，或者定语从句被分割时，用who.
I met a girl in the street yesterday who grew up in Yunnan province.
She did her best to help the others in the community who were deaf or blind.
D) 一个句子中带有两个定语从句时，其中一个定语从句的关系代词时that，另一个用who.
The boy that you met last night is the group leader who studies very hard.
E) 在there be开头的句子中，用who.
There is a young man who wants to see your father.
There was a king who was kind to his people.
There are many old men who are against this plan.
2) 关系代词whose的用法：关系代词whose是关系代词who的所有格形式，它既可以指人，也可以指物。
Whose n = the n. of which/whom = of which/ whom the n.
Do you know Peter whose father happens to be working in your company?
The tourist wanted to book a room whose window faces south.
3) 关系代词that和 which 用法
(1) 在限定性定语从句中指代物时，which和that一般可通用。
The new house that/which I has just bought is about six miles away.
They planted some trees that /which didn’t need much water.
(2) 只用that而不用which的情况。
A. 先行词本身是不定代词 few, all, none, little, something, anything, everything, nothing等，或者由不定代词every, all, any, much, little, few, no, one of, some修饰时
I’d like to tell you something that will make you surprised.
All that you need is a hammer and some nails.
We haven’t got much that we can offer you.
B. 先行词由the very, the only, the same修饰时
The only thing that we can do is to give you some money.
Li Ming is the only one that wants to be a teacher.
C. 先行词本身是序数词，形容词最高级或被其修饰时
This is the best way that has been used against pollution.
Polly is the most direct young woman that I have ever come across.
This is the third time that they have met.
The very first time that Tom saw the film, he made up his mind to become a doctor.
D. 先行词既有人又有物时（先行词=人 物）
He talked about the schools and teachers that he had visited.
The bike and his rider that had run over an old man were taken to the police station.
E. 主句是以who或which开头的特殊疑问句时
Which of the cars that are in front of the hotel belongs to you?
Who is the man that is sitting by the lake?
F. 主句是以here或there开头，且先行词指物时
Here is the car that I want to buy.
G. 有两个定语从句时，一个用which，另一用that
Edison built up a factory which produced things that had never been seen before.
(3) 只用which不用that
A．非限定语从句
The fish, which I bought this morning, is very fresh.
My house, which I bought two years ago, has got a lovely garden.
Jim passed the driving test, which surprised everybody in the office.
B．在限定性定语从句中，关系代词前有介词
I was put in a position in which I had to accept I was less important.
This is the hotel in which you will stay. = That’s the hotel (which/that) you will stay in.
此句中，如果介词in放在句子的后面，那么关系代词既可用which, 也可用that,还可省略
C．先行词后有插入语
You will see many rare wild animals, as I told you before, which attract many tourists in the Beijing Zoo.
D．有个两定语从句时，一用that，则另一用which
Let me show you the novel that I borrowed from the library which was newly open to us.
E．先行词本身是that
The clock is that which tells the time.
What's that which looks like a dog?
2. 关系副词：关系副词和关系代词一样，具有多重作用。由于关系副词在从句中均做状语，所以具体选用什么关系副词，主要取决于关系副词在从句中充当什么类型的状语。
1) 关系副词where的用法：有关系副词where引导的定语从句，其先行词主要是表示地点的名词，如building, city, room等，where在从句中作地点状语。
The hotel where we stayed was very clean. = The hotel (that/which) we stayed at was very clean.
2) 关系副词when的用法：有when引导的定语从句，其先行词主要是表示时间的名词，如time, day, date,等, when在从句中作时间状语。
I’ll never forget the time when we first met in London.
=I’ll never forget the time during which we first met in London.
The date (when/that) he joined the CPC was August 5.
从语法来角度讲，上例中的定语从句只能用when引导，但在口语中，常用that代替when, 而且还可以省略。
3) 关系副词why的用法：由why引导的定语从句，常用在先行词reason后面，why在句中作原因状语。
The reason why I got a job was that I worked hard.
=The reason (that)/for which I got a job was that I worked hard.
The reason why she was late was that she missed her plane.
=The reason (that)/for which she was late was that she missed her plane.
Amy didn’t get a pay rise, but this wasn’t the main reason why she left.
=Amy didn’t get a pay rise, but this wasn’t the main reason (that)/for which she left.
(一般说来，在定语从句中，只要先行词是the reason，它的关系副词就是why, 但在口语中，和when一样，why常被that代替，也可省略。)
重点比较：带reason的定语从句与表语从句的常用句型：
定语从句The reason why/that…;…the reason why/that…
表语从句The reason is that…(不能用why，否则就重复了)
I know the reason why she studies so well.
The reason is that he is always careless in his work.
四、关系代词和关系副词引导的定语从句的区别
对于同一个先行词，正确区分关系代词和关系副词，关键是要弄清楚它们在从句中承担什么成分，而不仅仅取决于先行词是什么。如果关系词在从句中只作主语或宾语，就是关系代词；如果关系词在定语从句中做状语，就是关系副词，当然上面提到的口语用法除外。
例句比较：1. This is the college (that/which) I visited.
2. This is the college where I studied three years ago.
3. I’ll never forget the day (that/which) we spent together.
4. I’ll never forget the day when I got married.
五、定语从句注意事项
1. 定语从句中的主谓一致
A) 定语从句中的关系代词作主语时，从句谓语的人称，数要与先行词一致。
Is he the man who/that wants to see you?
Is he or you who want to go to the zoo?
B) as/which作主语引导非限定性从句指全句时，从句谓语用第三人称。
Liquid water changes to vapor, which is called evaporation.
As is usual, Hans came to school late this morning.
C) 先行词为“one of 复数名词“时，关系代词为从句主语，谓语用复数。
Freddy is one of the students who want to be diplomats in our class.
D) 先行词为“the only one of the 复数名词“时，关系代词为从句主语，谓语用单数。
He is the only one of the students who has been a winner of scholarship for three years.
2. what, how不能用于定语从句中．
A）what不能用作关系代词来引导定语从句，但what引导的宾语从句可以与that引导的定语从句进行互换。
Tell me anything (that) you know. = Tell me what you know.
Tell me anything what you know. (错误)
B) how不能用作关系副词来引导定语从句，要用way作为先行词在从句中表达“方式”做状语，此时对应的关系词用which，that或省略。
This is how I worked out the problem.（表语从句）
This is the way how I worked out the problem. (错误)
This is the way (in which/that) I worked out the problem.（定语从句）
C) 关系代词作介词宾语时，如果介词出现在关系代词前，关系代词不可以省略；如果介词位于定语从句句末，关系代词可以省略。
This is the government building in which my father works.
This is the government (which/that) my father works in.
This is the government building in my father works. (错误)
D）there be句型之后的定语从句中，做主语或宾语的关系代词常省略。
There’s nothing (that) I can do about it.
3. 定语从句和同位语的区别
A) 根据that在从句中是否做成分来判定。引导定语从句的that是关系代词，在句中充当主语或宾语，是对先行词的一个说明。引导同位语从句的that是连接词，不充当任何成分，不能省略，也不能用which代替。同位语从句用来进一步说明名词或代词表示的具体内容。
The news that you told me last week is not true. (that引导定语从句，说明是哪一个news, that在从句中充当told的宾语。)
The news that the leader will come here is not true.
(that引导的同位语从句，说明news的具体内容，that在从句中不担任任何成分，但是不能省略，也不能被which代替)
B）根据意思来判断
在关系代词前加is后，如果句子成立就是同位语从句，否则就不是。
The news that the leader will come here is not true.
---The news is that the leader will come here. (成立，变成表语从句；是同位语从句)
The news that you told me is not true.
---The news is that you told me last week. (不成立；不是同位语从句)
六、说明方式时，不能用how（how不用于定语从句），要用“the way 关系代词”的方式来表达，具体如下：
“the way 从句"结构在英语教科书中出现的频率较高, the way 是先行词, 其后是定语从句。
具体看在定语从句中缺少的成分，根据缺少的成分，根据如下表格进行选择。
which | that | 省略 | in which | |
缺少主语 | ★ | ★ | ||
缺少宾语 | ★ | ★ | ★ | |
缺少状语 | ★ | ★ | ★ |
I don't like the way in which/that/（省略） you talk to me．做状语
I don't like the way which/that were taught by our teacher．----做主语
I don’t like the way which/that/ (省略) I just heard from the teacher. ----做宾语
七、as作为关系代词的用法
一）、as引导限制性定语从句，主要结构有：
the same…as
as…as
such…as
so…as
主句中出现the same, as, such, so修饰先行词，需选择as做关系代词，在定语从句中做主语或宾语。
1. It’s the same person as we wanted to find yesterday. 我们昨天要找的是同一个人。
2. Such girls as he knows are good at English. 他所认识的女孩都擅长英语。
3. Do you have such books as we like? 你有我们喜欢那种书吗？
4. She will marry as healthy a man as she can find. 她将嫁给她能找到的任何一个有钱人。
5. There is so warm a house as we want to live in. 这里有如此暖和的房子，我们都想住在里面。
6. He has so difficult a problem, as none of us can solve. 他有如此难的一个问题，我们没有人能解决。
二）、引导非限制性定语从句
⑴ as引导非限制性定语从句，指代整个主句内容，从句可置于句首，句中或句尾
1． As everyone knows, China is a beautiful country with a long history.
每个人都知道，中国是一个有着悠久历史的美丽国家。
2． The earth, as we know, moves round the sun. 地球，我们都知道，围着太阳转。
3． Tom works hard and is willing to help others, as we all know.
汤母工作努力，并且乐于助人，这一点我们都知道。
⑵ 非限制性定语从句中的谓语为被动式时，常用as做主语
be said/known/announced/reported/mentioned/expected/discussed
1. Grammar, as has been said before, is not a set of dead rules.
语法，就像以前所说，不是一套死规则。
2. As is known to all, Tai Wan is part of China.
⑶ as引导非限制性定语从句，有“如，似，正像”的含义，因此，下列句式多用as
as has been said above 如上所说
as anybody can see 正像每个人所看到的那样
as we had expected 正如我们所预料的那样
Things are not always as they appear. 事情并不一直像他们表面那样。
The boy has as much progress as we had expected.
正像我们所预料的那样，这个男孩取得了大的进步
八、重点提示
1、关系代词作主语时，从句谓语与先行词的人称和数一致
2、as, such作主引导非限定从，指全句时，从句谓语用单数（即把句子看作一个整体）
3、先行词为“one of the 复数n.”时，关系代词为从句主语，从句谓语用复数
4、先行词为“The only/very/right one of the 复数n.”时，关系代词为从句主语，谓语动词用单数
5、that, which, who在从句中做主语时，不能省略
6、关系代词whom, which在从句中做介词的宾语，且位于介词后面时，不可省略
7、先行词指人时，用who不用that的条件
8、先行词指物时，用that不用which的条件
9、只用which不用that的条件
10、as的用法
