在复合句中，(充当定语)修饰某一名词或代词的从句叫做定语从句。
被修饰的名词或代词叫做先行词。从句放在所修饰词(即先行词)之后。
定语从句由关系代词或关系副词引导。
关系代词和关系副词位于先行词和定语从句之间，它们的作用是：
1).连接主句和定语从句的作用。
2).指代先行词的作用。
3).在定语从句中充当一定成分。
关系代词引导的定语从句。
关系代词有：that, which, who, whom, whose.
The tall man who is playing basketball is Yao Ming.
正在打篮球的高个男人是姚明。
The girl that we saw yesterday is my sister.
我们昨天看到的女孩是我的妹妹。
This is the place which he wants to visit. 这是他想去参观的地方。
The boy whom I met last night is his brother.
我昨晚遇到的男孩是他的兄弟。
the boy whose mother is a teacher is Tom.
母亲是老师的那个男孩是汤姆。
如何选择关系代词来引导定语从句呢？主要依据先行词以及关系代词在定从中充当的成分来确定，如下表。
特别要注意。
1.关系代词在定语从句中做宾语时，常常可省略；作主语时，不能省略。
I don’t know the man (who) you saw last night. (作宾语)
The boy( whom) I met last night is his brother. (作宾语)
I like the book (that )you bought two days ago. (作宾语)
This is the place (which) he wants to visit. (作宾语)
2.先行词是物时，关系代词that可与which 互换。
I like the book which/that is written by him. (作主语)
This is the place (which/that) he wants to visit. (作宾语)
3.先行词是人时，关系代词that可与who 互换。
foreigners who/that visited our school yesterday are from Canada.(作主语)
I don’t know the man (who/that) you saw last night. (作宾语)
4.先行词是人，作宾语时，关系代词whom一般可与who、that 互换。
The boy (whom /who/that)I met last night is his brother. (作宾语)
He isn’t the man (whom/who/that) I am looking for. (作宾语)
5.关系代词whom和which在从句中做介词的宾语时，可以把介词放在关系代词
whom和which前面，即“prep + whom/which”位于先行词和从句之间，此时
关系代词不能省略；也可把介词放回从句中有关动词的后面，此时关系代词可
省略。
The beautiful lady to whom you just talked is Miss Zhang.
= The beautiful lady (whom/who/that) you just talked to is Miss Zhang.
刚才和你谈话的那位漂亮的女士是张老师。
The town in which he lives is far away from here.
他住的那个小镇离这儿很远。
=The town (which/that) he lives in is far away from here.
某些带有介词的动词短语属于固定搭配，拆开影响词义，此时介词仍放在动词
之后(即介词不可提前)。如：care for; look for/after;deal with;hear from;look forward to等。
This is the book (which/that)I’m looking for. 这正是我要找的那本书。
6.关系代词who、that作介词的宾语时，介词只能放在从句中有关动词的后面
(即介词不可提前)。
The friend(who/that/which)I am traveling with speaks English.
和我一起旅游的那个朋友说英语。
The town (that/which) he lives in is far away from here.
他住的那个小镇离这儿很远。
7.关系代词whose为关系代词who的所有格形式，用作名词的限定语；先行词
可为人，也可为物；“whose+名词”在定语从句中可作主语或(动词或介词的)宾语； “whose + 名词”可用“the +名词 of +whom/which”代替。
比如whose hometown=the hometown of whom; whose windows=the windows of which
This is the scientist whose name is known all over the world.
这是位世界著名的科学家。
I went to see my friends whose children I used to look after.
我去看望了我照顾过他们孩子的朋友。
The room whose window faces south is mine.窗户朝南的房间是我的。
He has written a book whose name I’ve forgotten.
他已写了一本我忘记了名字的书。
=He has written a book the name of which I’ve forgotten.
=He has written a book( which /that)I’ve forgotten the name of.
=He has written a book of which I’ve forgotten the name.
8.常用关系代词that而不用which的情况：
1）当先行词all,something,anything,nothing,everything,none,few,little,much
等不定代词时。
That’s all that I can do for you. 那是我能为你做的全部。
I would like to do something that is helpful to you. 我想做对你有益的事。
2）当先行词被形容词最高级、序数词以及all、last、any、some、only、few、
little、much、 no、very、every、same等修饰时。
This is the best hotel that I know. 这是我知道的最好的旅馆。
This was the first letter that I have received from her.
这是我从她那收到的第一封信。
This is the only food that I can find for you.
这是我能为你找到的仅有的食物。
All the books that he has translated are very popular here.
他翻译的所有书在这都很受欢迎。
3）当先行词有人又有物时。
I still remember the school and the teachers that I visited in Fuzhou last year.我仍然记得去年在福州参观的学校和拜访的老师们。
4)当主句是there be结构时，修饰主语的定语从句要用that引导
There is a book on the desk that belongs to Tom.桌子上的那本书是汤姆的。
5）在主句以疑问词who、which开头的疑问句中。
Which is the car that hit the boy? 撞了那个男孩的车是哪辆？
Who was the man that she danced with?和她跳舞的那个男人是谁？
9.常用who 而不用that的情况：
1）当先行词是指人的代词one、ones、anyone、someone、nobody或those等时。
Anyone who does that must be mad.谁要那样做肯定是疯了。
Those who dare to break the law will be punished.
任何人触犯了法律都会受到惩罚。
2）在there be结构中，先行词指人时。
There is a young man who wants to see you.有一个年轻人想要见你。
3）当先行词是I、you、he、they等时(常用于谚语中)。
He who plays with fire gets burned. 玩火者必*焚自**。
4）当先行词是指各个成员的集体名词时。
The family who live upstairs are fond of music.
住在楼上的那一家人喜欢音乐。
5）为了避免重复或引起歧义时。
The man that spoke at the meeting is our new headmaster who has come from Shanghai.
在会上发言的那个人是我们的新校长，他来自上海。
10.定语从句的谓语动词需要与先行词保持人称和数的一致。
1）先行词是单数，从句的谓语动词用单数形式；先行词是复数，从句的谓
语动词用复数形式。
I know the girl who draws well. 我认识那个很会画画的女孩。
There are some TV programs that are good for children.
电视上有些对孩子有益的节目。
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。
Lily is one of the students who were awarded.
丽丽是那些获奖的学生中的一个。
Lily is the only one of the students who was awarded.
丽丽是那些学生中唯一一个获奖的。