一、相同点

　　两者引导非限制性定语从句时，可指代主句中的部分或整个句子的内容，在从句中做主语，宾语或表语，位于主句之后有时可互换。

　　The meeting was put off, as /which was exactly what we wanted.

　　He was a doctor, as/which I knew from his manner.

　　二、不同点

　　1. as可以放在主句前后，也可插入主句之中;而which引导的非限制性从句只能位于主句之后。

　　As is known to all, fish can’t live without water.

　　Air, as we know, is gas.

　　2.在非限制性从句中，which后的be动词不可省;as而后的be动词可以省略。

　　She told me she won the match, which was a lie.

　　The material is elastic, as (was) shown in the figure.

　　3. which在非限制性定语从句做主语时可用各类动词做谓语;而as做主语时，谓语常用系动词，如be, seem, become等，一般不用其他行为动词。

　　He saw the girl, which delighted him.

　　He didn’t say anything at the meeting , as/which seemed very strange.

　　4. as常用“正如”含义，常用的结构有as we know(众所周知);as often happens(正如常发生的那样);as is often the case(情况常常如此); as we all can see(正如我们看到的); be announced/expected/known/imagined/mentioned/said/shown/

　　reported等。这些结构常放句首，偶尔也可以放句中或句末。

　　As is known to all, China is a developing country.

　　Kate was late for school, as often happened.

　　5. 在非限制性定语从句中which指代主句中某一个单词时，as不可以。

　　My brother enjoyed playing basketball, which he really plays well.

　　Beijing, which he was born in, is our capital.

　　6. “介词+关系代词(介宾代物)”中关系代词只能用which。

　　The Travel Agency, with which our company has been dealing for several years, has opened for new branches.

　　Air is a mixture of gases, of which oxygen forms 21 percent.

　　7. 从句中的谓语动词是否定形式时，常用which。

　　He can write a letter in English, which I can not.

　　Metal will bear beating with a hammer, which a stone will not

　　8.在非限制性定语从句关系代词代表前面主句中的宾语从句时，常用which

　　He said he had passed the exam, which was untrue.

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