考向二　as，which引导的定语从句

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，在从句中作主语、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.会议被推迟了，这正是我们想要的。

☛He was a doctor, as/which I knew from his manner.他是一名医生，我是从他的举止中知道的。

二、不同点

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

☛As is known to all, fish can’t live without water.众所周知，鱼离开水无法生存。

☛Air, as we know, is gas.正如我们所知，空气是气体。

2. 在非限制性定语从句中，which后的be动词不可省；而as后的be动词可以省略。

☛She told me she won the match, which was a lie.她告诉我们她赢得了比赛，这是一个谎言。

☛The material is elastic, as （was） shown in the figure.这种材料是有弹性的，正如图中所示。

3. which在非限制性定语从句中作主语时可用各类动词作谓语；而as作主语时，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.他看见了那个女孩，这让他很高兴。

☛He didn’t say anything at the meeting, as/which seemed very strange.他在会议上一言未发，这看起来非常奇怪。

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.众所周知，中国是一个发展中国家。

☛Kate was late for school, as often happened.凯特上学迟到了，正如常发生的那样。

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

☛My brother enjoyed playing basketball, which he really plays well.我哥哥喜欢打篮球，他打得非常好。

☛Beijing, which he was born in, is our capital.北京，他出生的地方，是我们的首都。

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.那家与我们交易了好几年的旅行社又开了新的分店。

☛Air is a mixture of gases, of which oxygen forms 21 percent.空气是一种气体的综合体，其中氧气占了21%。

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.他可以用英语写信，我不能。

☛Metal will bear beating with a hammer, which a stone will not.金属能承受得住用锤子敲打，但是石头则不能。

8. 非限制性定语从句中的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.他说他通过了考试，这是假的。