在复合句中作定语，修饰主句中某一名词或代词的从句，叫做定语从句。从句在复合句子中的作用相当于形容词，故又称为形容词性从句。
一. 先行词
被定语从句修饰的名词、代词称为先行词。先行词一般出现在定语从句之前。
Do you know the girl who is singing on the stage?
你认识在舞台上唱歌的那个姑娘吗?( the girl为先行词)
This is the museum which was built last year.
这就是去年建成的博物馆。( the museum为先行词)
I’ll forever cherish the days when I studied abroad.
我将永远怀念我在国外求学的日子。( the days为先行词)
Let’s find a place where we can have a picnic.
我们找一个能野餐的地方吧。( a place为先行词)
二. 关系词
引导定语从句的词被称为关系词。关系词根据其在定语从句中的功能可分为关系代词和关系副词。关系代词在从句中作主语、宾语、表语、定语等，关系副词在从句中作状语。
Speaking of the USA, New York is the first thing that comes to mind.
说到美国，人们最先想到的就是组约。(作主语)
I’ll cherish the moment when I met Lisa.
我将怀念我和丽莎见面的那一时刻。(作状语)
1. 关系代词
常见的关系代词如下表
（1）who, whom, that
He is the man who/that lives next door.
他就是住在隔壁的那名男子。 (who/that在定语从句中作主语，指代the man)
Mr White invited many friends to his party (who/whom/that) he respected.
怀特先生邀请了很多他尊敬的朋友参加他的聚会。(who/whom/that在定语从句中作respected的宾语，指代 many friends，可省略)
注意：
1）以下两种情况在定语从句中作宾语只能用whom，且不可省略：介词后宾语的关系代词只能用whom；在非限制性定语从句中，只能用whom作宾语。
I have many friends to whom I am going to send postcards.
我有很多要给他们寄贺卡的朋友。（whom在定语从句中作介词to后面的宾语）
A young man had a new girl friend, whom he wanted to impress.
一个年轻的小伙子新交了一个女朋友,他想给她留下深刻印象.（非限制性定语从句中，作宾语用whom）
2）宜用who不宜用that的情况
① 先行词是指人的不定代词时，如果先行词是one, ones, anyone时。
The ones who flatter me don't please me,
那些奉承我的人并不能取悦我
We don't want anyone who can't afford the tuition fees to drop out of school.
我们不想让任何一个交不起学费的人辍学。
②先行词为those时。
Those who have good manners will be highly respected.
那些有礼貌的人会备受尊敬。
③当先行词指人并含有较长的后置定语或在被分割的定语从句中时。
Professor Smith is coming soon who will give us a talk on how to learn English.
史密斯教授不久就要来给我们作一场关于如何学英语的讲座。
The boy was crying bitterly who lost his way while looking for his mother.
这个男孩在找妈妈的过程中迷了路，哭得很伤心。
④在 there be开头的句子中。
There is an old man who wants to see you.
有位老人要见你。
There are many young men who are against him.
有许多年轻人反对他。
（2） which, that
The river which/that runs through the centre of the city brings us lots of pleasure.
穿过市中心的那条河给我们带来很多欢乐。（which/that在定语从句中作主语）
This is the book (which/that) you are looking for.
这正是你在找的书。（which/that在定语从句中作宾语，可省略）
注意：
1) 指物时只能用which不能用that的两种情况，且不能省略。
① 在定语从句中作介词的宾语。
The house in which I used to live has become a shoeshop.
我过去住的房子现在变成一家鞋店了。
The pen with which you are writing is Jefr's.
你现在正用来写字的那支钢笔是杰夫的。
② 在非限制性定语从句中。
These apple trees，which I planted three years ago，have not borne any fruit.
这些苹果树是我三年前栽的，还没有结过果实。
2）在下列情况中，关系词指物时，只能用that引导，不能用 which引导。
①当先行词是all, little, much, none, everything, anything, nothing等不定代词时。
Is there anything that you don't understand?
你有什么不懂的地方吗?
Tom told his mother all that had happened.
汤却把发生的所有事情都告诉了他的妈妈。
②当先行词前面有 the only, the very, the last等修饰时。
This is the only book that I can find.
这是我能找到的唯一一本书。
③当序数词或形容词最高级修怖先行词时。
This is the best book that I have ever read.
这是我读过的最好的书。
④当先行词前面有only, all, any, no等修饰时。
I want to read all the books that were written by Lu Xun.
我想读所有鲁迅写的书。
⑤当主句是以疑问词 which开头的特珠疑问句时。
Which is the hotel that you like best?
哪个是你最喜欢的旅店?
⑥ 当先行词既有人也有物时。
They talked of things and persons that they remembered.
他们谈论他们记得的事和人。
3. whose
whose一般指人，但有时也指物，whose指代物时，相当于of which。在定语从句中作定语。whose指人时与of whom近似，但whose表示所有关系，而of whom则表示范围（主要用于非限制性定语从句。
The building whose roof（=the roof of which）we can see from here is a hotel.
我们dao从这儿能看见房顶的那座建筑是一家旅馆。
I’d like a room whose window (=the window of which) overlooks the sea.
我想要一个透过窗户能看到大海的房间。
Do you know the girl whose Japanese is excellent?
你认识那个日语非常优秀的姑娘吗?
He is the man whose car was stolen last night.
他就是昨晚车被偷的那个人。
I met the newcomers, several of whom were Chinese.
我见到了那些新来的人，他们中有几个是中国人。
注意：
关系代词的省略。一般情况下that， which，whom作宾语时可以省略，但当关系代词 which，whom在从句中作介词的宾语且位于介词后面时，不能省略。
I'll never forget the day on which I joined the club.
我永远不会忘记我加入俱乐部的那一天。
2. 关系副词
常见的关系副词如下表：
（1）when
when在定语从句中作时间状语。可由at/on/in/during which替代。
April Fool's Day is the day when (= on which ) people make fun ofothers.
愚人节是人们愚弄他人的日子。
Do you know the date when (= on which ) Lincoln was born?
你知道林肯的出生日期吗?
（2）where
where在定语从句中作地点状语，可由at/on/in/from/to which替代。
These are the few points where（= at which）we disagree with each other.
这些就是我们之间有分歧的几点。
This is the mountain village where (=in which) I stayed Iast year.
这就是我去年待过的山村。
He did all his research in the room where (=in which) he lived.
他在自己住的房间里做了所有的研究。
（3）why
why在定语从句中作原因状语，可以用 for which代替。why不可引导非限制性定语从句，此时可用for which。
I have come around to explain the reason why（=for which）I was absent from the meeting.
我来是为了解释我当时没有到会的原因。
Is this the reason why（=for which） she refused our offer?
这就是她拒绝我们帮助她的理由吗?
知识延伸
the reason why...与 the reason that
1)下面的句子是that引导的同位语从向，that不作成分，that后面的内容是解释说明 reason的内容的，而why引导的定语从向并不解释说明 reason的内容。
I don’t want to hear any reason why you were absent.
我不想听你缺席的任何理由。（why引导定语从句，在从句中作原因状语）
The reason that the car broke down dosen’t hold water.
车子坏了这一原因是站不住脚的。（that引导同位语从句，解释reason的内容，在同位语从句中不作成分）
2）在下面这个句子中，不能用why，只能用which或that。
I don’t want to hear any reason (that/which) you might give.
我不想听你可能给出的任何理由。(在这个定语从句中，that/ which作定语从句中谓语动词give的宾语)
注意：以下的关系副词可省略
1 ) the time, every time, each time, the moment后的关系副词可省略。
By the time (when) he was fourteen years old, Einstein had learnt advanced mathematics all by himself.
到十四岁时，爱因斯坦已经自学了高等数学。
2) 在某些表示地点的名词后，关系副词有时也可以省略，如 the place等。
This is the place ( where) we met years ago.
这就是我们多年前相遇的地方。
3) 先行词the reason后面的关系副词可省略。
The reason he did that is quite clear.
他那样做的理由是非常清楚的。
三. 定语从句中需注意的事项
- 主谓一致问题
关系代词在定语从句中作主语时，定语从句中谓语动词的单复数形式与先行词应一致。
（1）先行词是单数，从句的谓语动词用单数; 先行词是复数，从句的谓语动词用复数。
Mr Smith, who is now downstairs, is asking to see you
史密斯先生要见你，他现在在楼下。
The Smiths, who are now downstairs, are asking to see you.
史密斯夫妇要见你，他们在楼下。
（2） “one of+复数名词”后的定语从句的谓语动词用复数形式;＂the (only) one of+复数名词” 或“the very/ right one of+复数名词”后的定语从句的谓语动词用单数形式。
Jeff is one of the students who were awarded.
杰夫是那些获奖的学生中的一个。
Jeff is the only one of the students who was awarded.
杰夫是那些学生中唯一一个获奖的。
- what不可引导定语从句
what本身是“复合关系代词”，等同于“先行词+关系代词( which/that)”。因此不能用what引导定语从句。
Tell me what is happening. =Tell me all that is happening.
告诉我正发生着什么事。
She isn’t what she used to be. =She isn’t the one (that) she used to be.
她不是过去的她了。
She did all what she could to help the poor (x)
She did all ( that ) she could to help the poor. (V)
She did what she could to help the poor. (V)
地尽其所能帮助穷人。
3 .定语从句中关系代词与关系副词的选用
(1)用关系代词还是用关系副词完全取决于从句中的谓语动词。及物动词后面无宾语，就必须要用关系代词，而不及物动词则要用关系副词。
This is the village where/in which he was born.
这就是他出生的村庄。
This is the mountain village (which/that) he visited last year.
这就是他去年来过的村庄。
I will never forget the days when/during which I spent thesummer holidays in the countryside.
我将水远不会忘记我在乡下过暑假时的那些日子。
I will never forget the days (which/ that) I spent in the countryside.
我将水远不会忘记我在乡下度过的日子。
(2)看定语从句中缺少什么成分以确定是选用关系代词还是关系副词。如果定语从句中缺少主语、宾语、定语或表语，则这个关系词应是关系代词; 如果定语从句中缺少时间状语、地点状语或原因状语，那么我们应该选用关系副词。
Beijing is the place where(=in which)I was born.
北京是我的出生地。( where在从句中作地点状语)
Hele is the man whom/that I saw yesterday.
他就是我昨天看到的那个人。(whom/that在从句中作宾语)
