第2个回答 2017-01-15
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.
第3个回答 2017-01-15
as引导的定语从句表示说话人对话语的看法、态度、依据、解释或评论。具有下列特点：
1．as用于引导定语从句时，具有“正如……、像……、由……而知”等语义。如：
He was strongly against the plan，as could be expected．（正如所料，他强烈反对这项计划。）
As he hoped he would，he saw the girl．（像他希望的那样，他见到了那个女孩。）
2．as引导的定语从句在语义上要和主句保持一致，而which可不一定，如果语义不一致，则要用which来替代as。如：
He married again，as（which）was natural．（他又结婚了，这是自然的。）
3．as引导的定语从句，其先行词可由以下词来修饰，the same ／such ／as等。也可以由
which来改写，但不可直接替换。如：
This is the same sum as was spent in that year on private motoring．＝This is
the sum which was spent in that year on private motoring．（这与那年用于私人汽车所花的费用相同。）
4．as引导的非限制性定语从句可放在主句之后，也可放在主句之前。但which只能放在主句之后。如：
As we have seen，oceans cover more than 70％of the earth．（如我们所见，海洋覆盖地球70％多的面积。）
He seemed a foreigner，which in fact he was．（他看上去像个外国人，事实上他就是。）