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一、定语从句的概念：在复合句中，修饰某一名词或代词的从句叫定语从句。
例如： 1) The man who lives next to us is a fool.
2) You must buy me the house which is near Xiangshan.
上面两句中的the man和the house是定语从句所修饰的词，叫先行词，定语从句放在先行词的后面。
二、关系词(连接词)
1.关系代词引导的定语从句
引导定语从句的关系词有关系代词：that, which, who (宾格whom, 所有格whose) 和关系副词where, when, why。关系词放在先行词和定语从句之间，起连接作用，同时还做定语从句的一个成分。
2.关于that, which的用法注意点
1)只能用that，不用which作为定语从句关系代词的情况
a)不定代词，如anything, nothing, the one, all, much, few, any, little等作先行词
There is nothing that I can do.
I mean the one that was bought yesterday.
b)先行词有the only, the very, the same, the last修饰
This is the very book that I want to find.
The last place that I visited was the hospital.
c)先行词为序数词、数词、形容词最高级时
This is the first composition that he has written in English.
d)先行词既有人，又有物时
He talked about the teachers and schools that he had visited.
2)不用that, 只用which的情况
a)引导非限定性定语从句时which
The tree, which is four hundred years old, is very famous here.
b)介词后用which
We depend on the land from which we get our food.
3.关系副词引导的定语从句
关系副词可代替的先行词是时间、地点或理由的名词，在从句中作状语。
1)先行词是表示地点名词时(country, school, room…)，而关系词代替先行词在定语从句中充当地点状语，一般用where引导定语从句。也可以用介词+which的结构。值得注意的是which前的介词选择是根据先行词与介词的搭配关系而定。
Beijing is the place in which (=where) I was born.
He wants to find the place in which (=where) he lived forty years ago.
2)先行词是表示时间名词时(year, month, day, night…)，而关系词代替先行词在定语
从句中充当时间状语，一般用when引导定语从句。也可以用介词+which的结构。值得注意的是when前的介词选择是根据先行词与介词的搭配关系而定。
His father died that year in which (=when) he was born.
I cannot forget the first day on which (=when) my family moved into the city.
3)先行词是the reason，而关系词代替先行词在定语从句中充当原因状语，一般用why引导定语从句。也可以用for + which的结构。
Is this the reason for which (why) he refused our offer?
但是这里要指出的是，如果介词和Which搭配在一起并不表示以上三种意思，则我们只能保留介词+Which的形式。如 I know English in which I wrote the love letter.
4.判断关系代词与关系副词
方法一： 用关系代词，还是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
求用关系代词;而不及物动词则要求用关系副词。