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 ）
