一．定义：在复合句中，修饰名词或代词的从句；被修饰的名词或代词叫做先行词，引导定语从句的词叫关系词，关系词分为关系代词和关系副词；定义从句相当于形容词的用法。
二．关系副词：
1.作用：引导定语从句，指代先行词并且在从句中作成分，关系副词只能作状语；
2.分类：
①when，在定语从句中做时间状语；
②where，在定语从句中作地点状语；
③why，在定语从句中作原因状语;
④that，在定语从句中可作时间，地点，原因，方式状语。
3.关系副词可以转换为相应的介词加which，介词来源于先行词，固定搭配或是句意理解。
4. when引导的定语从句：
(1) when引导的定语从句修饰表示时间的先行词，并在从句中作时间状语；
(2)时间介词：in/on/at/after/before/for/since/during…
①in+超过一天的时间词（month/season/year/century等）；
e.g. in 2008 在2008年
in spring/summer/autumn/winter在春季/秋季/秋季/冬季
in April/October 在四月/十月
in the 21st century在二十一世纪
②on +具体到某一天的时间词（day/date等）;
e.g.on Monday/Sunday在星期一/在星期天
on the National Day在国庆节
on Sunday morning/afternoon/evening在星期天早上/下午/晚上
on September 1st在九月一号
③at+表几点几分的时间词（hour/minute等）；
e.g. at six thirty在6：30
at seven 在7：00
④特殊用法(固定搭配)：
e.g. in the morning/afternoon/evening在早上/下午/晚上
at night/noon/dawn/dusk在晚上/正午/黎明/黄昏
⑤after在……之后；
e.g.I didn’t see home after that day.
⑥before在……之前；
e.g. Tom had lived here before 1998.
⑦in +时间段在……之后；在……之内；
e.g. Mary will go to the USA in three days.
⑧for +时间段多长时间；
e.g. I have studied English for eight years.
⑨since+过去的年份，自从……
e.g. Tom has lived there since 1996.
⑩during +时间段，意为“在……期间”。
e.g. Tom stayed in Beijing during summer holiday.
(3)关系副词when可以换为时间介词：in / on / at / after / before / for / since / during…
+ which。
e.g. October 1, 1949 is the day when(=on which) the People’s Republic of China was founded.
I’ll never forget the time when(=during which) we worked on the farm.
Do you remember the afternoon when (=on which) we first met three years ago?
5. where引导的定语从句：
(1) where引导的定语从句修饰表示地点的先行词，并在从句中作地点状语；
(2)地点介词：in, at, on…
①in +表示大地点的地点名词（国家/城市等）；
e.g. in Beijing 在北京
in New York在纽约
in China在中国
②at +表示小地点的地点名词（机场/家等）；
e.g.at home在家里
at airport在机场
③特殊的地点介词用法：
e.g.on the farm在农场
(3)关系副词where可以换为地点介词：in / on / at… + which。
e.g. This is the place where (=at/in which) we first met.
The hotel where (=in which) we stayed wasn’t clean.
I recently went to the town where (=in which) I was born.
I’d like to live in a country where (=in which) there is plenty of sunshine.
6. why引导的定语从句：
(1) why引导的定语从句修饰表示原因（reason）的先行词，并在从句中作原因状语；
(2)原因介词：for;
(3)关系副词why可以换为原因介词: for + which。
e.g. I didn’t get a pay rise, but this wasn’t the reason why (=for which) I left.
The reason why (for which) he was late was that he missed his train.
7. that引导的定语从句：that作关系副词常代替引导定语从句的关系副词when, where和why, 而且往往可以省略。
e.g. We’ll find some good place(that) we’ll have a picnic.
He didn’t tell me the reason(that)he was so upset.
That was the day (that) he wrote the report.
8.注意：time作先行词，如前面有序数词或last修饰时，不能用when引导定语从句，要用that或省去不用。但是，如果time之前无序数词或last，则when和that皆可引导定语从句。
e.g. This is the last time that I shall give you a lesson.
The first time(that) I entered the university was in 1988.
9. 注意：当先行词是表示方式的the way时，定语从句通常不用how来引导，应该用that或in which,或将它们全部省略。
e.g. We admired the way he solved the problem.
I don’t like the way (that/in which)he eyed me.
In general, the way people think about themselves has a profound effect on all areas of their life.
10. 限制性定语从句：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。这种从句和主句的关系十分密切，不用逗号隔开。并且做宾语和表语的关系代词可以省略。
e.g. What is the name of the man who just came in?
Beijing is a city that I always want to visit.
He found the book that (that) he was looking for.
She met a man (that/whom/who)she met on a bus.
This was the time when the two countries were at war.
11. 注意：在限定性定语从句中，在某些表示时间，地点的词后，关系副词在口语中有时可以省略。
e.g. That was the year (when) I first went abroad.
We need a place (where) we can stay for a few days.
12.关系副词why引导的定语从句的先行词只有reason，why可转换为for which， 在口语中可用that或者省略。
e.g. The reason (why/for which/that)I bought the flowers is that Mary likes them.