一、定语从句的定义
在复合句中， 修饰名词或代词的从句叫做定语从句。被定语从句所修饰的词叫先行词。引导定语从句的词叫关系词。关系词包括关系代词who(宾格whom,所有格whose),that, which 和关系副词where, when, why 等。
二、定语从句的种类
定语从句可以分为限定性定语从句和非限定性定语从句。
1.限定性定语从句：它是先行词在意义上不可缺少的定语，如果去掉，主句的意思就不完整或失去意义。
The man that wears a red hat is my uncle.
2.非限定性定语从句：它只是对先行词作附加补充说明，如果去掉，主句的意思仍然清楚完整。书写时往往用逗号分开。非限定性定语从句一般不用that 引导，而且不可以省略关系词。
The old woman, who lives on her own, has a cat for company.
Beijing, which is the capital city of China, is a very beautiful city.
三、关系代词的用法
在定语从句中， 关系代词起着连接主句与从句，指代先行词和在从句中作句子成分的三重作用。关系代词的选用取决于主句中的先行词以及关系代词在从句中作何种成分。
（一）关系代词who, whom 的用法
1.who 可以代替人（即它的先行词必须是人），在从句中担任主语，往往也可以代替在从句中担任宾格的whom，但它的前面不能有介词，如果带介词则必须用宾格whom，即“介词+whom”.
Jim is a person who always has novel ideas.
The person who/whom you just talked to is Mr. Depp.
The person to whom you just talked is Mr. Depp.
（二）关系代词whose 的用法
关系代词whose 是关系代词who 的所有格形式，它既可以代人，也可以代物。当whose 代物时，相当于of which。
Do you know Peter whose father happens to be working in your company?
The tourist wanted to book a room whose window faces south.
=The tourist wanted to book a room the window of which faces south.
=The tourist wanted to book a room . The room’s window faces south.
（三）关系代词that, which 的用法
（1）在限定性定语从句中指代物时，which 和that 一般可通用。
The new house that/which I has just bought is about six miles away.
（2）限定性定语从句中只用that 而不用which 的情况。
A）先行词有形容词最高级修饰时
This is the best way that has been used against pollution.
B) 先行词有序数词修饰时
This is the third time that they have met.
C)主句已有疑问词who 或which 时
Which of the cars that are in front of the hotel belongs to you?
D) 先行词既有人又有物时
He talked about the schools and teachers that he had visited.
E) 先行词为all, much, few, little, something, everything, anything, nothing, none 等不定代词时
It’s easy to do the repair. All that you need is a hammer and some nails.
四、关系副词的用法（where, when, why）
关系副词在从句中均做状语，所以具体选用什么关系副词，主要取决于关系副词在从句中充当什么类型的状语。
（一）关系副词where 的用法
由关系副词where引导的定语从句，其先行词主要是表示地点的名词，如building, city, room等，where 在从句中作地点状语。
The hotel where we stayed was very clean.= The hotel (that/which) we stayed at was very clean.
（二）关系副词when 的用法
由when引导的定语从句，其先行词主要是表示时间的名词，如 time, day, date,等，when 在从句中作时间状语。
I’ll never forget the time when we first met in London .
=I’ll never forget the time during which we first met in London.
（三）关系副词why 的用法
由why 引导的定语从句，常用在先行词reason 后面，why 在句中作原因状语。
The reason why I got a job was that I worked hard.
五、关系代词和关系副词引导的定语从句的区别
如果关系词在从句中制作主语或宾语，就是关系代词；如果关系词在定语从句中做状语，就是关系副词。例句比较：
This is the college (that/which) I visited.
This is the college where I studied three years ago.
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