众所周知，which与as都可以引导定语从句，但它们有许多用法上的区别，现就此简要分析如下：
一、which与as引导非限制性定语从句的区别：
1．Which只能放在主句后面，而as引导的从句位置相对较灵活，可在前，可在中，也可在后。例如：
A) As he realized, I was very useful to him.(在前)
B) Air, as we know, is a gas.（在中）
C) He was a foreigner,as I know from his accent.(在后)
2.在which引导的定语从句中，谓语动词若是am, is, are则这些系动词不能省略；而as后面若是这种情况，则可以省略。例如：
A) He is a teacher, as (is) clear from his manner.(is可以省略)
B) He said he had never seen her before, which was not true.(was不可省略)
3.当which引导的从句在意义上近乎并列关系时，可以用and this，and that代替，意思是“这件事”例如：
A) He changed his mind, which(and this, and that)made me very angry.他改变了主意，这使我很生气。
B) She has married again, which(=and this, and that)was unexpected.出乎意料的是，她又结婚了。
而as主要起与上下文连接的作用，表达说话人的看法、观点，并指出主句内容的出处或根据等，例如：
C）Einstein, as we know, is a famous scientist.爱因斯坦，众所周知，是位伟大的科学家。
D）As is announced in today’s newspaper, we must improve our style of work.今天的报纸上说，我们必须改进工作作风。
4．作主语时，which既可以作系动词be的主语，也可以作实义动词的主语；而这时as 只可以做系动词be的主语。例如：
A) He married her, which was natural.（可用as代替which）他和她结婚，这是很自然的事。
B) He won a/the Nobel Prize in chemistry, which made him illustrious. (不可用as 代替which)他获得了诺贝尔化学奖，这使他名扬天下。
5．当定语从句位于主句之后，纯粹表示主句所述内容，不带有“如……那样”的意思时，往往用which,而不用as。例如：
A) They were invited to the state banquet, which(as)was a great honor to them.他们被邀请参加国宴，这对他们来说是莫大的荣幸。
B) We had to sleep in our wet clothes, which was most uncomfortable.我们不得不穿着湿衣服睡觉，这简直太不舒服了。
C) Mummy always treats me just like a baby, which I can’t bear.妈妈老把我当成小孩对待，这让我无法忍受。
6．当定语从句有“如同……那样”含义时，用as比用which更常见，而这些从句几乎成为固定说法。例如：
A) Things will turn out contrary to one’s wishes, as is often the case.事与愿违，这是常用的事。
B) As was natural, this inordinate hope was followed by an excessive depression.这种过分希望之后，接着是极度的沮丧，是很自然的事。
C) Chaucer is buried in “poet’s Corner”,as might have been expected.正如人们已经预料的，乔叟被葬在“诗人角”。
D) The material is elastic, as is shown in the figure.这种霉烂有弹性，如图所示。
E) As we know(众所周知)
F) As has been said above/before（正如前文所述）
G) As has been pointed out（正如已经指出的）
H) As might be imagined（可以想象得到）
7．当定语从句的关系代词指的是先行词本身时，只能用which。例如：
A) These apple trees, which I planted three years ago, have not borne any fruit.这些苹果树是我三年前种下的，没结出任何果实。
B) The Thames, which is now clean enough to swim in, was polluted for over a hundred years.泰晤士河，现在已经干净，可以在里面游泳，它曾被污染了一百多年。
8．带介词的典型的定语从句，必须用which，不能用as。例如：
A) There is a great deal of oxygen around us, without which we could not live.我们周围有许多氧气，没有它我们就无法生存。
B) The shed in our garden, in which we often played, has lasted for a long time.我们经常玩耍的花园里的那个棚子，已经有很长时间了。
二、引导限制定语从句时which与as的区别：
1．先行词如为表示物的名词或代词，在从句中又做介词的宾语时，只能用which。例如：
This is the pan in which I boiled the milk.这就是我煮牛奶的锅。
2．前面有as时，引导定语从句的关系代词用as,不用which。例如：
There are as many dictionaries as are needed.所需要的字典都有了。
3．前面有such时，引导定语从句的关系代词用as，不用which。
1）Such women as know Tom thought he was charming.认识汤姆的女人都认为他很迷人。
2）I have never heard such stories as he tells.他讲的那些故事我从没有听过。
3）He is not such a man as I expected.他不是我期望的那种人。
1．前面有the same时，引导定语从句的关系代词用as而不用which。例如：
1）We are facing the same problems as we did years ago.我们现面临着和多年以前同样的困难。
2）This is the same wallet as I lost.这只钱夹子与我丢失的那只相同。
3）I have the same trouble as you have.我和你有着同样的困难。
总之，which与as引导定语从句的区别要在实践中多体会，体会多了，才能正确运用。
补充：
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引导的定语从句只表示一个众所周知或意料之中的事，但which不受此限制。即as所引导的从句所表达的语意须和主句一致
She has married again，which was unexpected.她又结婚了，真不可思议。这是出乎意料的(此处不可用 as)
