考研英语语法点睛：as 和 which 的 区别
as 和 which 都可用作关系代词来引导从句，它们两个到底有什么不同呢，这是同学们经常觉得困惑的地方。下面详细讲讲它们之间的联系和区别，希望能解开大家心中的结。
一、as, which 指代整个句子或短语。
由as, which 引导的非限定性定语从句，相当于and this或and that。在很多时候，表示说话人对话语的看法或态度，此时它们的先行词并不是具体的某个词，而是整个句子或短语：
The Chinese government has decided to develop the west of China, which, I dare say, will benefit the people there, especially those who are still leading a poor life. 中国政府决定开发西部，我敢说，这将造福于西部的人民，特别是那些至今过着贫穷生活的人。
She has married again，which delighted us.她又结婚了，这使我们很高兴。
John said he’ d been working in the office for an hour，which was true.
The result of the experiment was very good， which we hadn’t expected.
Carol said the work would be done by October，which personally I doubt very much.
二、as, which 用在句末的时候，有些情况下可以互换使用。
He was late for school，as/which was usual with him.他上学迟到，这是家常便饭。
He was a foreigner，as/which I knew from his accent.他是个外国人，我从他的口音中得知这一点。
I live a long way from work，as/which you know.我离上班的地方很远，你是知道这一点的。
The author was brought up in a small village, as/which is recounted in some of his stories. 作者是在一个小村庄里长大的，这个问题在他的一些小说了就被提到过。
但是，当主句是否定句时，只能as。例如：
Spiders are not insects, as many people think. 蜘蛛不是昆虫，而许多人却认为蜘蛛是昆虫。
三、as 的特殊用法。
1、如果要说明两者有相似之处时通常用as，表达“正如(情形或事实所表示)的那样、像……的一样”的意思。例如：
She is a fine singer，as her mother used to be.她是一个很出色的歌唱家，和她母亲当年一样。
She is the same age as you (are). 她和你年龄一样大。[与same...as 连用]
I hope to get such a dictionary as he is using. 我希望有一本和你正在用的一样的词典。[与such...as连用]
As water is to fish, so air is to man. 正如水对鱼一样重要，空气对于人同样重要。
She has married again，as was/seemed natural.她又结婚了，这很自然。
2、as常与从句中的know，see， hear，expect等动词连用，也常用于as often happens，as is often the case(常有的事)等句子中。
As you know, he is from Beijing. 正如你所知道的，他来自北京。
As is known to everybody，the moon travels round the earth once every month.
He was absent，as is often the case.他没有上学，这是常事。
As you will find out, all is now settled. 你将会看到这样的情况，一切都已搞定了。
3、as引导的非限制性定语从句的位置比较灵活，既可位于所修饰的句子之后，也可位于句中或句首；而由 which引导的非限制性定语从句一般不位于句首。例如：
As you will find out，I will never let you down.你将会发现，我绝不会使你失望的。
4、as所引导的从句所表达的语意须和主句一致，而 which则无此限制。例如：
She has married again，which was unexpected.她又结婚了，真不可思议。(此处不可用 as)
