which與as引導(dǎo)非限制性定語從句的異同

　　一、相同點(diǎn)

　　兩者引導(dǎo)非限制性定語從句時(shí)，可指代主句中的部分或整個(gè)句子的內(nèi)容，在從句中做主語，賓語或表語，位于主句之后有時(shí)可互換。

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

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

　　二、不同點(diǎn)

　　1. as可以放在主句前后，也可插入主句之中;而which引導(dǎo)的非限制性從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.在非限制性從句中，which后的be動(dòng)詞不可省;as而后的be動(dòng)詞可以省略。

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

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

　　3. which在非限制性定語從句做主語時(shí)可用各類動(dòng)詞做謂語;而as做主語時(shí)，謂語常用系動(dòng)詞，如be, seem, become等，一般不用其他行為動(dòng)詞。

　　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”含義，常用的結(jié)構(gòu)有as we know(眾所周知);as often happens(正如常發(fā)生的那樣);as is often the case(情況常常如此); as we all can see(正如我們看到的); be announced/expected/known/imagined/mentioned/said/shown/

　　reported等。這些結(jié)構(gòu)常放句首，偶爾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. 在非限制性定語從句中which指代主句中某一個(gè)單詞時(shí)，as不可以。

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

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

　　6. “介詞+關(guān)系代詞(介賓代物)”中關(guān)系代詞只能用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. 從句中的謂語動(dòng)詞是否定形式時(shí)，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.在非限制性定語從句關(guān)系代詞代表前面主句中的賓語從句時(shí)，常用which

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