一、 限制性与非限制性定语从句
(一) 限制性定语从句与非限制性定语从句的区别
限制性定语从句对先行词起修饰限制作用;而非限制性定语从句是对先行词起补充说明作用。非限制性定语从句中，先行词与定语从句往往有逗号割开。非限制性定语从句相当于并列句、状语从句等。如：
I want this man, who (=for he) can speak English.
He gave up the plan, which (=though it) was a very good one.
I met John, who (=and he) told me the news.
I will take this one, which (=for it) seems to be the best one.
He has two sons, who work in the same company. (He has only two sons.)
He has two sons who work in the same company. (Perhaps he has more than two sons.)
(二) 非限制性定语从句中关系代词和关系副词的用法
1、 关系代词和关系副词在任何情况下都不能省略;
2、 Who(主语)，whom(宾语)，which(主语、宾语)不能用that代替，也不能互相替换;
3、 介词+which/whom+从句结构中，介词不能移到从句的后面;
4、 when, where可用于非限制性定语从句。
二、 关系代词的用法
(一) 关系代词的作用和分类
1、关系代词的作用有三个：
(1) 连接作用：关系代词引导从句，把它和主句连接起来;
(2) 替代作用：关系代词在从句中替代它前面的先行词;
(3) 成分作用：关系代词在从句中总是充当句子成分。
2、关系代词的用法分类
关系代词的用法与分类有三点依据：
(1) 根据所引导的从句的限制性和非限制性;
(2) 根据所替代的先行词是指人还是指物;
(3) 根据它在从句中所充当的成分----主语、宾语、表语或者定语。
(二) 关系代词that和which的用法
1、 限制性定语从句中，必须用关系代词that的情况：
(1) 当先行词是不定代词all, much, little, something, everything, anything,
nothing, the one时。如：
Do you have anything that you want to say for yourself?
You should hand in all that you have.
(2) 当先行词前面被the only, the very, any, few, little, no, all等词修饰时。
如：
This is the very person that I’m waiting for.
The only thing that we can do is to give you some money.
(3) 当先行词是形容词最高级或者先行词的前面有形容词最高级修饰时。如： This is the best that has been used against pollution.
This is the most interesting film that I’ve ever seen.
(4) 当先行词是序数词或它前面有序数词修饰时。如：
This train is the last that will go to Suzhou.
What is the first American film that you have seen?
(5) 当先行词既有人又有物时。如：
Do you know the things and persons that they are talking about?
(6) 当主句的主语是疑问词who或which时。如：
Which is the bike that you lost?
Who is the boy that won the gold medal?
(7) 有两个定语从句时，其中一个关系代词也在从句中作表语时。如：
They secretly built up a small factory, which produced things that could cause pollution.
(8) 当先行词在主句中作表语，而关系代词也在从句中作表语时。如：
Shanghai is no longer the city that is used to be.
2、 定语从句中，必须用which的情况：
(1) 在非限制性定语从句中，只用which不用that。如:
Helen was much kinder to her youngest son than to the others, which, of course, made the others envy him.
(2) 当动词短语中的介词提前时，只用which不用that。如：
This is a house in which Lu Xun once lived.
注意：在一些固定搭配的`动词短语中，由于动词和介词不可分割，因此把介词置于关系代词之前。如：
This is the pen (which/that) I’ m looking for.
(三) 关系代词who, whom和whose的用法
先行词指人：在定语从句中作主语时，用who,不克省略;在定语从句中作宾语时，用whom/that,可以省略;在定语从句中坐定语时，用whose，不可省略。如：
She is the girl who lives next door.作主语
That’s the girl (whom/that) I teach.作宾语
That’s the scientist whose achievements are well known.作定语
This is the house whose window broke last night.
= This is the house, the window of which broke last night.
= This is the house, of which the widow broke last night.
(四)“介词+关系代词”引导的定语从句
“介词+关系代词”引导定语从句时，关系代词只能用which或whom，即：介词+whom/which。
1. 当介词放在关系代词的前面时，关系代词只能用which或whom，关系代词不能省略。如：
(1) Is this the house in which Shakespeare was born?
(2) He paid the boy 10 Yuan for washing ten windows, most of which hadn’t
been cleaned for at least a year.
(3) In the dark street, there wasn’t a single person to whom she could turn
for help.
(4) Recently I bought an ancient vase, the price of which was very
reasonable.
(5) The gentleman about whom you told me yesterday proved to be a thief.
2.在限制性定语从句中，当介词位于定语从句的末尾时，可用that/which ,that/whom/who作介词的宾语，而且作介词宾语的关系代词可以省略。如：
3.”复合介词短语+关系代词which”引导的定语从句，这种结构引导的定语从句常与先行词用逗号分开，定语从句常用倒装语序。如：
He lived in a big house, in front of which stood a big tall tree.
4.介词+which/whom+不定式结构
The poor man has no house in which to live.
= The poor man has no house to live in.
= The poor man has no house in which he can live.
