用来说明主句中某一名词或代词（有时也可说明整个主句或主句中一部分）而起定语作用的句子叫作定语从句。
一．关系代词引导的定语从句
1. 关系代词用来指代先行词是人或物的名词或代词
例１：This is the detective who came from London.
例２：The book which I am reading is written by Tomas Hardy.
例３：The desk whose leg is broken is very old.
例４：This is the room that Shakespeare was born in.
２．关系代词的用法
(1) 如果先行词是all, much, anything, something, nothing, everything, little, none等不定代词，关系代词一般只用that，不用which。例如：
All the people that are present burst into tears.
(2) 如果先等词被形容词最高级以及first, last, any, only, few, mush, no, some, very等词修饰，关系代词常用that,不用which, who，或whom。例如：
(3) 非限制性定语从句中，不能用关系代词that,作宾语用的关系代词也不能省略。例如：
There are about seven million people taking part in the election, most of whom、are well educated.
(4) which还有一种特殊用法，它可以引导从句修饰前面的整个主句，代替主句所表示的整体概念或部分概念。在这种从句中，which可以作主语，也可以作宾语或表语，多数情况下意思是与and this 相似，并可以指人。例如：
He succeeded in the competition, which made his parents very happy.
(5) that可指人或物，在从句中作表语，（指人作主语时多用who）仅用于限制性定语从句中。
(6) which可作表语，既可指人，以可指物。指人时，一般指从事某种职业或是有种特征．品性或才能的人。Which引导的定语从句可以限制性的，也可以是非限制性的。
(7) 如果作先作词的集体名词着眼于集体的整体，关系代词用which;若是指集体中的各个成员，则用who。
(8) 先行词有两个，一个指人，一个指物，关系代词应该用that。例如：
The boy and the dog that are in the picture are very lovely.
(9) 如果先行词是anyone, anybody, everyone, everybody, someone, somebody,关系代词应该用 who 或whom，不用 which。例如：
Is there anyone here who will go with you?
３．“介词＋关系代词“是一个普遍使用的结构
(1) “介词＋关系代词“可以引导限制性定语从句，也可以引导非限制性定语从句。“介词＋关系代词“结构中的介词可以是 in, on, about, from, for, with, to at, of, without 等，关系代词只可用whom或 which,不可用 that 。
(2) from where为“介词＋关系副词“结构，但也可以引导定语从句。例如：
We stood at the top of the hill, from where we can see the town..
(3) 像listen to, look at, depend on, pay attention to, take care of等固定短语动词，在定语从句中一般不宜将介词与动词分开。例如：
This is the boy whom she has taken care of.
二．关系副词引导的定语从句
1．关系副词也可以引导定语从句
关系副词在从句中分别表示时间．地点或原因。关系副词when在从句中充当时间状语，where 充当地点状语，why充当原因状语。
2. that可引导定语从句表示时间．地点或原因
That有时可以代替关系副词 when, where 或者why引导定语从句表示时间．地点或原因，在 that引导的这种定语从句中，that也可以省去。
三．限制性定语从句与非限制性定语从句
1．二者差异比较
限制定语从句紧跟先行词，同先行词这间一般不加逗号，仅修饰先行词，可以由关系代词．关系副词或that来引导。非限制性定语从句仅作补充或说明，用逗号与主句隔开，既可修饰先行词，又可修饰整个主句，不可用that引导。
2．关系代词和关系副词的选择依据
(1) 弄清代替先行词的关系词在从句中作什么成分,作状语的应选用关系副词，作主语．宾语或表语的可选用关系代词。
3. 先行词与定语从句隔离
定语从句一般紧跟在先行词之后，但定语从句与先行词之间有时也会插入别的成分，构成先行词与定语从句的隔离。例如：
1) This is the article written by him that Is poke to you about..
2) He was the only person in this country who was invited
四．As在定语从句中的用法
1. 引导限制性定语从句和非限制性定语从句
（１）as多与such 或the same连用，可以代替先行词是人或物的名词。
（２）as 也可单独使用，引导非限制性定语从句，作用相当于which。例如：
The elephant’s nose is like a snake, as anybody can see.
（３）the same… that与 the same …as在意思上是不同的。
2．As引导的非限制性定语从句的位置
as引导的非限制性定语从句位置较灵活，可以位于主句前面．中间或后面，一般用逗号与主句隔开，但which所引导的非限制性定语从句只能放在主句之后。例如：
(1) As is expected, the England team won the football match.
(2) The earth runs around the sun, as is known by everyone.
五．学习定语从句的几个问题
定语从句又称为关系从句，是最常见的从句之一，每年高考题对之均有考查。
(一)、定语从句中关系代词与先行词的一致性
定语从句中关系代词一般应与它所指代的先行词的单复数保持主谓一致。例如：
The students who were here just now are from No,2 Senior Middle School.
但注意下列一组句子：
He is the only one of the students who was here just now.
He is one of the students who were here just now.
如果"one of +复数名词"后跟有定语从句，一般情况下"one of"后的复数名词为先行词，但当one前有the only,the very,just the修饰时，先行词则为one。
(二)、定语从句与强调结构
It is the place where they lived before.
It is in the place that they lived before.
第一个句子为定语从句，where指代the place,在定语从句中作状语，第二个句子为强调结构，强调in the place,that没有意义，把in the place 放回后面句子，句子意思完整。
Where is it that he found the lost watch?(强调句型，强调疑问副词where)。
Where is the watch that he found yesterday.(定语从句，that指代the watch)
(三)、定语从句与并列结构
He has two sons,neither of whom looks like him.
He has two sons,and neither of them looks like him.
I've got two sisters.Both of them are in Shanghai.
第一个句子为定语从句，关系代词whom指代two sons,在定语从名中介词of的宾语。第二个句子为并列结构，由并列连词and连接，人称代词them指代two sons。第三个为两个独立的句子，两个句子中间用句号，两句开头的处一个字母都大写。
(四)、定语从句与状语从句
He found the books where he had put.
He found the books in the place where he had put.
第一个句子为状语从句，where he had put 作主句He found the books 的地点状语。第二个句子为定语从句，where引导从句修饰the place。
This is such an interesting book that I'dlike to read it.
This is such an interesting book as I'd like to read.
第一个句子为结果状语从句，在结果状语从句中，it指代book,作read的宾语。第二个句子为定语从句，关系代词as指代先行词book的定语从句中read的宾语。
(五)、定语从句中的先行词
Is this book the one that you bought yesterday?
Is this the book that you bought yesterday?
第一个句子中，this book是主句的主语，the one 是先行词。在第二个句子中this是主句的主语，the book是先行词。一定要避免出现：Is this book that you bought yesterday?
(六)、定语从句与同位语从句
定语从句相当于形容词，它对先行词起修饰、描述或限制作用，而同位语从句则相当于名词，它对其前面的词给予说明或作进一步解释，即说明该词所表示的具体内容。例如：
The news that we heard is not true.(定语从句)
The news that he won the prize is not true.(同位语从句)
另：在"have no idea+从句"结构中，其从句都作idea的同位语。例如：
I have no idea when she will be back.
六、定语从句易犯小错误
由于定语从句的结构和用法比较复杂，初学者在使用时往往容易犯一些错误，最常见的有如下七种：
(一)、在定语从句中加了多余的定语。如：
1．误：Some of the boys I invited them didn’t come．
正：Some of the boys I invited didn’t come．
译：我邀请的男孩中有几个没有来。
析：应删去them，因为从句的宾语是省略了的whom，who或that。
2．误：The book that you need it is in the library．
正：The book that you need is in the library．
译：你需要的书在图书馆里。
析：应删去it，因为从句的宾语是关系代词that。
(二)、把定语从句谓语动词的单、复数弄错。如：
1．误：Anyone who break the law will be punished．
正：Anyone who breaks the law will be punished．
译：任何违犯法律的人将被处罚。
析：应改break为breaks，因为who指anyone，是单数。
2．误：Those who has finished may go home．
正：Those who have finished may go home．
译：做完了的人现在可以回家。
析：应改has为have，因为who指those，是复数。
3．误：He is the only one of the teachers who know French in our school．
正：He is the only one of the teachers who knows French in our school．
译：他是我们学校中唯一懂法语的人。
析：应改know为knows，因为one前有the only之类限定词，定语从句在意义上修饰的是the only one，是单数，而不是复数名词the teachers。
4．误：This is one of the rooms that is free now．
正：This is one of the rooms that are free now．
译：这是目前空着的房间之一。
析：应改is为are，因为one前没有the only之类的限定词，定语从句在意义上修饰的是名词复数the rooms，而不是单数one。
(三)、误省略了定语从句中作主语的关系代词。如：
1．误：Children eat a lot of sugar often have bad teeth．
正：Children who that eat a lot of sugar often have bad teeth．
译：吃糖多的孩子往往牙齿不好。
析：应加上关系代词who或that，因为从句少主语，且主语不能省略。
2．误：The key opens the bike is missing．
正：The key that／which opens the bike is missing．
译：开这辆自行车的钥匙不见了。
析：应加上关系代词that或which，因为从句少主语，且主语不能省略。
(四)、定语从句中加了多余的关系副词或介词。如：
1．误：The house where he lives in needs repairing．
正：The house where he lives needs repairing．
或：The house he lives in needs repairing．
译：他住的房子需要修理。
析：应保留where，删去从句中的in，因为关系副词where在从句中作地点状语，in属多余。或删去关系副词where，因为where在这里的意思是in which，否则介词in就重复了。
2．误：I still remember the day on when I first came to Beijing．
正：I still remember the day when I first came to Beijing．
或：I still remember the day on which I first came to Beijing．
译：我仍记得我第一次来到北京那天的情景。
析：应删去on，因为when在这里的意思是on which，否则介词on就重复了，或把when改为which。
(五)、在作先行词的时间名词或地点名词后错用了关系代词或关系副词。
1．误：I still remember the day when we spent together．
正：I still remember the day that／which we spent together．
译：我仍记得我们在一起度过的日子。
析：应改when为that或which，因为从句中谓语动词spent是及物动词，其后应跟宾语而不是时间状语。
2．误：This is the house where we lived in last year．
正：This is the house which／that we lived in last year．
译：这是我们去年住过的那个房子。
析：应改where为which或that，因为从句谓语动词lived后有介词in，其后少介词宾语，而不是地点状语。
(六)、在先行词reason后错用关系副词why。如：
1．误：Have you asked her for the reason why may explain her absence？
正：Have you asked her for the reason that／which may explain her absence？
译：你是否向他问过可以解释他缺席的原因？
析：应改why为that或which，因为定语从句缺少主语，而不是少原因状语。
2．误：I don’t believe the reason why he has given for his being late．
正：I don’t believe the reason that／which he has given for his being late．
译：我不相信他所提供的他迟到的原因。
析：应改why为that或which，因为从句谓语动词has given后缺少宾语，而不是缺少原因状语。
(七)、误将强调句型当定语从句。如：
1．误：It was in the kitchen where the fire broke out．
正：It was in the kitchen that the fire broke out．
译：大火发生在厨房。
析：应将where改that，因为原句还原为The fire broke out in the kitchen后，在语法和句意上均成立，故此题是强调句型，而非定语从句。
2．误：Was it because it snowed last night when you didn’t come？
正：Was it because it snowed last night that you didn’t come？
译：你是否因昨晚下雪而没有来？
析：应将when改为that，因为，原句还原为Because it snowed last night，you didn"t come后，在语法和句意上均成立，故此题是强调句型而非定语从句。
七．定语从句的注意事项
1.一般说来，除了用定语从句解释名词或泛指外，先行词前应有定冠词the。
2.在限制性定语从句中which,whom,that充当宾语时，可以省略。而在非限制性定语从句中whom,who,which不能省略。
3.在含有非限制性定语从句的复合句中从句与主句之间应该用逗号隔开。
4.定语从句与同位语从句的区别。首先看引导从句的关系词that是否在从句中充当句子成分。如果在从句中充当主、宾、表，则是定语从句。再看that前的名词是否是一些需要有内容的名词，如：idea,fact,thought,news等。后面的从句是说明其内容的，（这个名词在后面的从句中部充当任何成分），这个从句则是同位语从句。He expressed the hope that he has had for many years.
hope在从句中不充当句子成分，又加上后面从句是说明hope的内容的，因此时同位语从句。
5.the reason why(=for which)是由why或for which引导的定语从句。而the reason that也是that引导的定语从句。that往往省略。
This is the reason why he was late. =This is the reason (that)he was late.
6.当主句中有who,which时，而定语从句中也要用到who或which时，为了避免who…who,which…which等重叠，定语从句用that引导。
Who is the man that is standing by the door?
Which of the two cows that you keep produces more milk?
7.先行词为人和物作并列成分时，定语从句用that引导。
John and his dog that were here a moment ago disappear now.
8.不论人或物在定语从句中作表语时，用that引导定语从句，而that通常省掉。
He is no longer the man that he was.
9.“one of＋可数名词复数”引导的定语从句中,谓语动词应用复数；而“one of＋可数名词复数”前有the，only或the only修饰，定语从句中的谓语动词用单数。
He is one of the students who study very hard at school.
He is the(only/the only) one of the students who studies very hard at school.
1.一般说来，除了用定语从句解释名词或泛指外，先行词前应有定冠词the。
2.在限制性定语从句中which,whom,that充当宾语时，可以省略。而在非限制性定语从句中whom,who,which不能省略。
3.在含有非限制性定语从句的复合句中从句与主句之间应该用逗号隔开。
4.定语从句与同位语从句的区别。首先看引导从句的关系词that是否在从句中充当句子成分。如果在从句中充当主、宾、表，则是定语从句。再看that前的名词是否是一些需要有内容的名词，如：idea,fact,thought,news等。后面的从句是说明其内容的，（这个名词在后面的从句中部充当任何成分），这个从句则是同位语从句。He expressed the hope that he has had for many years.
hope在从句中不充当句子成分，又加上后面从句是说明hope的内容的，因此时同位语从句。
5.the reason why(=for which)是由why或for which引导的定语从句。而the reason that也是that引导的定语从句。that往往省略。
This is the reason why he was late. =This is the reason (that)he was late.
6.当主句中有who,which时，而定语从句中也要用到who或which时，为了避免who…who,which…which等重叠，定语从句用that引导。
Who is the man that is standing by the door?
Which of the two cows that you keep produces more milk?
7.先行词为人和物作并列成分时，定语从句用that引导。
John and his dog that were here a moment ago disappear now.
8.不论人或物在定语从句中作表语时，用that引导定语从句，而that通常省掉。
He is no longer the man that he was.
9.“one of＋可数名词复数”引导的定语从句中,谓语动词应用复数；而“one of＋可数名词复数”前有the，only或the only修饰，定语从句中的谓语动词用单数。
He is one of the students who study very hard at school.
He is the(only/the only) one of the students who studies very hard at school.
