導語：定語從句不同於單詞作定語的情況，它通常只能放在被修飾的詞之後。下面小編告訴大家英文中什麼是定語從句，歡迎參考。
在複合句中 ， 修飾名詞或代詞的從句叫定語從句 ， 被修飾的名詞或代詞叫先行詞 ， 引導定語從句的有關係代詞 who, whom, whose, which, that等和關係副詞where, when, why等 ， 關係代詞和關係副詞在定語從句中擔任句子成份。
定語從句公式：定語從句=先行詞+關係詞+從句
1.由who引導的定語從句中 ， who用作主語 ， 如 ： This is the boy who often helps me.
2.由whom引導的定語從句中，whom用作賓語，如：The man whom you are waiting for has gone home.
3.由whose引導的定語從句中 ， whose用作定語 ， 如 ： Do you know the girl whose skirt is white?
4.由which引導的定語從句中，which用作主語或謂語動詞的賓語或介詞的賓語，如：
The room in which there is a machine is a work shop.
The river which is in front of my house is very clean.
This is the pen which you want.
注意 ：
(1)whom, which用作介詞賓語時 ， 介詞可放在 whom、which之前 ， 也可放在從句原來的位置上;但在含有介詞的動詞固定片語中，介詞只能放在原來的位置上。如： He is the very person whom we must take good care of.
(2)引導非限制性定語從句時，必須用關係代詞which，不用that,如：I have lost
my bag, which I like very much.
(3)關係代詞在句中作主語時，從句的.謂語動詞的人稱和數必須和先行詞保持一致。
5.由that引導的定語從句中，that可以指人或物，在從句中作主語或謂語動詞的賓語，但不能放在介詞後面作介詞賓語，如：
The book that I bought yesterday was written by Lu Xun.
注意在下面幾種情況下必須用 that引導定語從句。
(1)先行詞是不定代詞all, few, little, much, something, nothing, anything等 ， 如 ：
All that we have to do is to practise English.
(2)先行詞被序數詞或形容詞最高階所修飾，如
The first letter that I got from him will be kept.
(3)先行詞被all, any, every, each, few, little, no, some等修飾 ， 如
I've eaten up all the food that you gave me.
(4)先行詞被the only, the very, the same, the last修飾時如
He is the only person that I want to talk with.
(5)先行詞既有人又有物時，如:
They talked about persons and things that they met.
(6)當句中已有who時 ， 為避免重複 ， 如 ： Who is the man that is giving us the class?
6.由when, where, why引導的定語從句，如:
I don't know the reason why he was late.
This is the place where we have lived for 5 years.
I'll never forget the day when I met Mr Li for the first time.
注意：先行詞是表示地點時，如果從句的謂語動詞是及物的，就用 that(which)，如果從句的謂語動詞是不及物的，就用where引導。This is the house Which /that he has lived in for 15 years.(Where he has lived for 15 year.)
7. 限制性定語從句和非限制性定語從句
(1)限制性定語從句是句中不可缺少的組成部分，主句和從句之間不用逗號分開。引導非限制性定語從句的關係代詞有who, whom, whose, which, of which等，這些關係代詞都不能省略。
(2)非限制性定語從句是對主句先行詞的補充說明 ， 沒有這種從句 ， 不影響主句意思的完整 ， 一般用逗號把主句和從句分開 ， 關係代詞用 which,不用that;指人時可用who,如 ： I have two brothers, who are both students.
8.如何簡化定語從句
(1).定語從句簡化為形容詞或形容詞短語作後置定語。如:
My grandfather lives in a village that is far away from here.
→My grandfather lives in a village far away from here.我祖父住在離這兒很遠的一個村子。
This is a book that is worth reading.
→This is a book worth reading. 這是一本值得看的書。
(2) 定語從句簡化為現在分詞或現在分詞短語作前置或後置定語。
The man who is standing under the tree is our English teacher.
→The man standing under the tree is our English teacher.
站在樹下面的那個人是我們的英語老師。
I saw the house that was burning at that time.→I saw the burning house at that time.
當時我看到那房子在燃燒。
(3)定語從句簡化為過去分詞短語作後置定語。
I like to see the films which are directed by Zhang Yimou.→I like to see the films directed by Zhang Yimou. 我喜歡看張藝謀導演的電影。
She is the girl who was praised at the school meeting.→She is the girl praised at the school meeting. 她就是在校會上受表彰的那個女孩。
(4)定語從句簡化為不定式作後置定語。
He is always the first person that comes to school.→He is always the first person to come to school.他總是第一個到校。
The report which will be given tomorrow is important to us.→The report to be given tomorrow is important to us. 明天要作的報告對我們很重要。
(5)定語從句簡化為what 從句。
I couldn't remember the words that he said.→I couldn't remember what he said.
我記不得他說的話。
9.關係代詞引導的定語從句
關係代詞所代替的先行詞是人或物的名詞或代詞，並在句中充當主語、賓語、定語等成分。關係代詞在定語 從句中作主語時，從句謂語動詞的人稱和數要和先行詞保持一致。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?
他就是想見你的人嗎?(who/that在從句中作主語) He is the man whom/ that I saw yesterday.
他就是我昨天見的那個人。(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：
They rushed over to help the man whose car had broken down. 那人車壞了，大家都跑過去幫忙。
Please pass me the book whose (of which) cover is green. 請遞給我那本綠皮的書。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. 農村出現了前所未 有的繁榮。(which / that在句中作主語)
The package which / that you are carrying is about to come unwrapped. 你拿的包快散了。(which / that 在句中作賓語)
10.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、地點或理由的名詞，在從句中作狀語。1)when, where, why
關係副詞when, where, why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. 任何人都有不得不屈服的時候。
Beijing is the place where (in which) I was born. 北京是我的出生地。
Is this the reason why (for which) he refused our offer? 這就是他拒絕我們幫助他的理由嗎?2)that代替關係副詞
that可以用於表示時間、地點、方式、理由的名詞後取代when, where, why和"介詞+ which"引導的定語從 句，在非正式文體中that常被省略(正式文體中不可省略)，例如：
His father died the year (that / when / in which) he was born. 他父親在他出生那年逝世了。
He is unlikely to find the place (that / where / in which) he lived forty years ago. 他不大可能找到他四十 年前居住過的地方。
