定语从句，起形容词的作用，在句中常用来修饰名词或代词。被修饰的词称为先行词,引导定语从句的词称为关系词, 关系词的作用一是放在先行词与定语从句中间起引导作用;二是在意义上代替先行词,并在从句中充当一个成分。其中关系代词：who, whom, whose, which, that, as;关系副词：when, where, why。
eg. She is the girl who sings best of all.
The pen which my uncle gave me is missing.
He lives in a house whose windows face south.
The factory where my father works is in the east of the city.
Perhaps the day will come when people will be able to breathe clean air in cities.
1. 先行词是人，作主语，关系代词用who, that
eg. He is a man( ) never leaves today’s work till tomorrow.
The boy ( ) is standing there is my cousin.
2. 先行词是人，作宾语，关系代词用 whom, who, that,
eg. Here is the man ( ) you’ve been expecting to meet.
The man ( ) you met yesterday is Mr. Smith.
3. 先行词是物，作主语，关系代词用which, that
eg . The train ( ) has just left is for Guangzhou.
Children like to read books ( ) have wonderful pictures.
4. 先行词是物，作宾语，关系代词用which, that,或省略
eg. The book ( ) you borrowed yesterday is really interesting.
The pen ( ) my uncle gave me is missing.
eg. He is the professor ( ) name was Jackson.
China, ( ) population is the largest in the world, is developing very fast.
eg. I can’t remember the date ( ) he went abroad.
I’ll never forget the day ( ) I joined the army.
2. 先行词是表示地点的名词，在定从中作地点状语，关系代词用where; 其中注意表示抽象概念的地点名词，如，situation、stage、degree、point等表示方面或程度时，也需用where
eg. This is the village ( ) Uncle Wang once lived.
They have reached the point ( ) they have to separate with each other.
He’s got himself into a dangerous situation ( ) he is likely to lose control over the plane.
eg. I don’t know the reason ( ) he was late.
None of us know the reason ( ) Tom was absent from the meeting.
4.引导定语从句的关系副词也可以用“适当介词 + which”来代替。
eg. October 1, 1949 was the day when ( = ) the People’s Republic of China was founded.
This is the factory where(= ) we worked a year ago.
I don’t believe the reason why (= ) he was late for school.
Ⅳ. 关系代词that & which的区别:
① 先行词为all, everything, anything, nothing, little, much等不定代词时。
eg. There is nothing ( ) can prevent him from doing it.
② 先行词被any, only, few, no, very, little 等修饰时。
eg. This is the very book ( ) I’m looking for.
eg. The first place ( ) they visited in Guilin was Elephant Trunk Hill.
This is the best film ( ) I have ever seen.
eg. He talked about things and persons ( ) they remembered in the school.
⑤ 先行词被the only, the very修饰时。
eg. Mr. Smith is the only foreigner ( ) he knows.
eg. Who is the man ( ) is standing beside Tom?
⒉ 不能用 that的情况：
eg. He had failed in the maths exam , ( ) made his father very angry.
② 介词 + 关系代词。
eg. This is the room in( ) my father lived last year.
Ⅴ. as引导定语从句时的用法(as 相当于that & which)
① as引导限制性定语从句通常用于the same … as, such … as结构中。
eg. This is the same book ( ) I lent you.
Such machines ( ) are used in our workshop are made in China.
② as引导非限制性定语从句既可放在主句之前，也可放在主句之后，位置灵活，用来修饰整个句子。通常用下列句型：as is known to all, as is said, as is reported, as is announced, as we all know, as I expect 等。
eg. ( ) I expected, he got the first place again in this mid-term examination.
Taiwan, ( ) we all know, belongs to China.
eg. I was the only person in our office ( ) was invited.(去掉定语从句，意思就不完整)
Tom’s father, ( ) is over sixty, still works hard day and night.(整个句子可分成两句来翻译)
eg. There is an expression in his eyes ( )I can’t understand.
I was the only person in my office ( ) was invited to the important ball.
I suggest you choose someone I think ( ) is very kind and friendly.