定語從句(Attributive Clauses)在句中做定語，修飾一個名詞或代詞，有時也可以修飾部分或整個句子。
被修飾的名詞，詞組或代詞即先行詞。定語從句通常出現在先行詞之後，由關係詞(關係代詞或關係副詞)引出。
關係代詞有：who, whom, whose, that, which, as。
關係副詞有：when, where, why, how。
關係代詞和關係副詞放在先行詞和定語從句之間，起連接作用，同時又可做定語從句的一個成分。當關係代詞做賓語時可以省略。
定語從句中的謂語動詞必須在人稱上和數量上和先行詞保持一致。
定語從句分為限制性定語從句和非限制性定語從句。
1
關係代詞引導的定語從句
1) who, whom, that 這些詞代替的先行詞是人的名詞或代詞，在從句中所起作用如下：
Is he the man who/that wants to see you?
(who/that在從句中作主語)
He is the man whom/ that I saw yesterday.
(whom/that在從句中作賓語)
2) whose 用來指人或物，(只用作定語，若指物，它還可以同of which互換).
例如：
Please pass me the book whose (of which) cover is green.
3) which, that 它們所代替的先行詞是事物的名詞或代詞，在從句中可作主語、賓語等.
例如：
A prosperity which / that had never been seen before appears in the countryside.
(which / that在句中作賓語)
The package (which / that) you are carrying is about to come unwrapped.
(which / that在句中作賓語)
關係代詞that和which 都可以指物，that 和Who 都可以指人，其用法區別：
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不用that的情況：
a) 在引導非限定性定語從句時
(錯)The tree, that is four hundred years old, is very famous here.
b) 介詞後不能用
We depend on the land from which we get our food.
c) 多用who 的情況
①關係代詞在從句中做主語
A friend who helps you in time of need is a real friend.
②先行詞為those, people 時
Those who were either fools or unfit for their offices could not see the cloth.
③先行詞為all, anyone, ones, one 指人時
One who doesn't work hard will never succeed in his work.
④在There be句型中There is a stranger who wants to see you.
⑤在被分隔的定語從句中
A new teacher will come tomorrow who will teach you German.
⑥在有兩個定語從句的句子中，其一用who，其二用that,但若先行詞後接兩個以上的並列定語從句時，後一個必須重複前一個關係代詞。
The student who was praised at the meeting is the monitor that is very modest and studies very hard.
There is a teacher who is always ready to help others and who enjoys what he does.
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只能用that作為定語從句的關係代詞的情況
a)在不定代詞，如：anything, nothing, everything, all, much, few, any, little等作先行詞時，只用that，不用which。
All that is needed is a supply of oil.
Finally, the thief handed everything that he had stolen to the police.
b)先行詞有the only, the very, the just修飾時，只用that。
He is the very man that helped the girl out of the water.
c)先行詞為序數詞(the last)、數詞、形容詞最高級時，只用that。
The first English book that I read was "The Prince and the Pauper" by Mark Twin.
d)先行詞既有人，又有物時。
He talked about the teachers and schools that he visited.
e)當主句是以who 或which 開始的特殊疑問句時，用that 以避免重複。
Who is the person that is standing at the gate.
f)關係代詞在從句中做表語
He is not the man that he used to be.
3
關係副詞引導的定語從句
關係副詞可代替的先行詞是時間、地點、方式或理由的名詞，在從句中作狀語。
關係副詞when, where, why, how的含義相當於"介詞+ which"結構，因此常常和"介詞+ which"結構交替使用.
例如：
There are occasions when (on which) one must yield.
Beijing is the place where (in which) I was born.
Is this the reason why (for which) he refused our offer?
I'm surprised the way how (by which) he works out the problem.
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注意：
①在非限制性定語從句中，"介詞+ which"結構不能代替關係副詞。
如：They set up a state for their own , where they would be free to keep Negroes as slaves.
②含有介詞短語的動詞一般不能拆開，介詞仍放在動詞後面。
Is this the book which (that) she was looking for?
3
名詞/數詞/代詞 /形容詞最高級 + 介詞 + 關係代詞引導定語從句
She has written a book , the name of which I have forgotten.
There are fifty-five students in our class , all of whom are working hard.
There are five continents in the world , the largest of which is Asia.
4
as, which 引導非限定性定語從句的差別
由as, which 引導的非限定性定語從句，as和which可代整個主句，相當於and this或and that。As一般放在句首，which在句中。
As we know, smoking is harmful to one's health.
The sun heats the earth, which is very important to us.
as可引導非限制性從句，常帶有「正如」的意思。
As is know, smoking is harmful to one's health.
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用法區別：
(1) as 引導的定語從句可置於句首，而which不可。
As we all know, he never smokes.
(2) as 代表前面的整個主句並在從句中作主語時，從句中的謂語必須是系動詞;若為行為動詞，則從句中的關係代詞只能用which。
(3)非限定性定語從句中出現expect, think, suppose 等表示猜測、想像、預料等時。
She succeeded in her doing the research work , as we expected.
(4)As 的用法 the same? as; such?as 中的as 是一種固定結構, 和??一樣??。
I should like to use the same tool as is used here.
We should have such a dictionary as he is using.