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1. 所有关系代词that，who ，whom， which， whose 引出的定语从句
关系代词修饰先行词，语法上要充当从句得主语，宾语，定语等。
非正式文体中，作宾语得关系代词常省略;作主语得在句型为：it be……， that be …… ， there be……时才省略。
从句的谓语要与修饰语保持一致。
This is the book whiich/that has just appeared.
The girl (whom/that) you saw yesterday is my girlfriend.
Then he met Mary， who nivited him to a party.
Here is Alice， (whom) I mentioned the other day.
若先行词是everything， something， anything， none， one， some， little， much， the only， the very， 高级等，有人和物组成时，用that或省略;人时用who or whose.
Is that everything that you want to tell us
This is the only property that i have.
He is the greatest man that has ever lived in that country——这个that作主语在这里不能省略，请注意!
There are only three boys and two cars that i can see.
2. 关系代词as ， but 引出的定语从句
as 可用在the same……as， such……as， as……as……等结构中，作主语，宾语，表语，状语等成分。
but 此时= that/who/which ……not. 与带有否定词的主句连用，在从句中一般作主语。
In the nuclear power station we use the same generator as is used in the thermal power station.
Ice consists of the same nolecules as water (consist of)
I have never seen such kind of people as they are.
There is no rule but has (which/that does not have) exception.
3.关系副词when， where， why 引出得定语从句
when， where 句中作状语，=“介词 which"
修饰reason可用why or for which or that
在非正式文体中，when and why 常省略，where 被介词后置的形式代替。
I do not remember the day (when/on which) he left.
The year (when/in which) I entered the university was 1908.
With TV， we can see things happen almost at the exact moment (when/that/at which) they are happpening.
That is the rason (why/that/for which) she spoke.

4. as， which 引导的特种定语从句
在引导非性定语从句时，as，which 代表前面整个句子。
as 可作主语，宾语，作主语时谓语动词用第三人称单数，位置不定。
which 作主语， 同上，引导的从句不可前置。
As was usual at the weekend， the club was almost empty.
As you can see， we have got a problem with the engine.
Metals have many good propertics， as has been stated before.
He missed the train， which annoyed him very much.