眾所周知，which與as都可以引導定語從句，但它們有許多用法上的區別，現就此簡要分析如下：
壹、which與as引導非限制性定語從句的區別：
1．Which只能放在主句後面，而as引導的從句位置相對較靈活，可在前，可在中，也可在後。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.(在後)
2.在which引導的定語從句中，謂語動詞若是am, is, are則這些系動詞不能省略；而as後面若是這種情況，則可以省略。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.當which引導的從句在意義上近乎並列關系時，可以用and this，and that代替，意思是“這件事”例如：
A) He changed his mind, which(and this, and that)made me very angry.他改變了主意，這使我很生氣。
B) She has married again, which(=and this, and that)was unexpected.出乎意料的是，她又結婚了。
而as主要起與上下文連接的作用，表達說話人的看法、觀點，並指出主句內容的出處或根據等，例如：
C）Einstein, as we know, is a famous scientist.愛因斯坦，眾所周知，是位偉大的科學家。
D）As is announced in today’s newspaper, we must improve our style of work.今天的報紙上說，我們必須改進工作作風。
4．作主語時，which既可以作系動詞be的主語，也可以作實義動詞的主語；而這時as 只可以做系動詞be的主語。例如：
A) He married her, which was natural.（可用as代替which）他和她結婚，這是很自然的事。
B) He won a/the Nobel Prize in chemistry, which made him illustrious. (不可用as 代替which)他獲得了諾貝爾化學獎，這使他名揚天下。
5．當定語從句位於主句之後，純粹表示主句所述內容，不帶有“如……那樣”的意思時，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.他們被邀請參加國宴，這對他們來說是莫大的榮幸。
B) We had to sleep in our wet clothes, which was most uncomfortable.我們不得不穿著濕衣服睡覺，這簡直太不舒服了。
C) Mummy always treats me just like a baby, which I can’t bear.媽媽老把我當成小孩對待，這讓我無法忍受。
6．當定語從句有“如同……那樣”含義時，用as比用which更常見，而這些從句幾乎成為固定說法。例如：
A) Things will turn out contrary to one’s wishes, as is often the case.事與願違，這是常用的事。
B) As was natural, this inordinate hope was followed by an excessive depression.這種過分希望之後，接著是極度的沮喪，是很自然的事。
C) Chaucer is buried in “poet’s Corner”,as might have been expected.正如人們已經預料的，喬叟被葬在“詩人角”。
D) The material is elastic, as is shown in the figure.這種黴爛有彈性，如圖所示。
E) As we know(眾所周知)
F) As has been said above/before（正如前文所述）
G) As has been pointed out（正如已經指出的）
H) As might be imagined（可以想象得到）
7．當定語從句的關系代詞指的是先行詞本身時，只能用which。例如：
A) These apple trees, which I planted three years ago, have not borne any fruit.這些蘋果樹是我三年前種下的，沒結出任何果實。
B) The Thames, which is now clean enough to swim in, was polluted for over a hundred years.泰晤士河，現在已經幹凈，可以在裏面遊泳，它曾被汙染了壹百多年。
8．帶介詞的典型的定語從句，必須用which，不能用as。例如：
A) There is a great deal of oxygen around us, without which we could not live.我們周圍有許多氧氣，沒有它我們就無法生存。
B) The shed in our garden, in which we often played, has lasted for a long time.我們經常玩耍的花園裏的那個棚子，已經有很長時間了。
二、引導限制定語從句時which與as的區別：
1．先行詞如為表示物的名詞或代詞，在從句中又做介詞的賓語時，只能用which。例如：
This is the pan in which I boiled the milk.這就是我煮牛奶的鍋。
2．前面有as時，引導定語從句的關系代詞用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時，引導定語從句的關系代詞用as，不用which。
1）Such women as know Tom thought he was charming.認識湯姆的女人都認為他很迷人。
2）I have never heard such stories as he tells.他講的那些故事我從沒有聽過。
3）He is not such a man as I expected.他不是我期望的那種人。
1．前面有the same時，引導定語從句的關系代詞用as而不用which。例如：
1）We are facing the same problems as we did years ago.我們現面臨著和多年以前同樣的困難。
2）This is the same wallet as I lost.這只錢夾子與我丟失的那只相同。
3）I have the same trouble as you have.我和妳有著同樣的困難。
總之，which與as引導定語從句的區別要在實踐中多體會，體會多了，才能正確運用。
補充：
壹、as, which 指代整個句子或短語。
由as, which 引導的非限定性定語從句，相當於and this或and that。在很多時候，表示說話人對話語的看法或態度，此時它們的先行詞並不是具體的某個詞，而是整個句子或短語：
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. 中國政府決定開發西部，我敢說，這將造福於西部的人民，特別是那些至今過著貧窮生活的人。
She has married again，which delighted us.她又結婚了，這使我們很高興。
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 用在句末的時候，有些情況下可以互換使用。
He was late for school，as/which was usual with him.他上學遲到，這是家常便飯。
He was a foreigner，as/which I knew from his accent.他是個外國人，我從他的口音中得知這壹點。
I live a long way from work，as/which you know.我離上班的地方很遠，妳是知道這壹點的。
The author was brought up in a small village, as/which is recounted in some of his stories. 作者是在壹個小村莊裏長大的，這個問題在他的壹些小說了就被提到過。
但是，當主句是否定句時，只能as。例如：
Spiders are not insects, as many people think. 蜘蛛不是昆蟲，而許多人卻認為蜘蛛是昆蟲。
三、as 的特殊用法。
1、如果要說明兩者有相似之處時通常用as，表達“正如(情形或事實所表示)的那樣、像……的壹樣”的意思。例如：
She is a fine singer，as her mother used to be.她是壹個很出色的歌唱家，和她母親當年壹樣。
She is the same age as you (are). 她和妳年齡壹樣大。[與same...as 連用]
I hope to get such a dictionary as he is using. 我希望有壹本和妳正在用的壹樣的詞典。[與such...as連用]
As water is to fish, so air is to man. 正如水對魚壹樣重要，空氣對於人同樣重要。
She has married again，as was/seemed natural.她又結婚了，這很自然。
2、as常與從句中的know，see， hear，expect等動詞連用，也常用於as often happens，as is often the case(常有的事)等句子中。
As you know, he is from Beijing. 正如妳所知道的，他來自北京。
As is known to everybody，the moon travels round the earth once every month.
He was absent，as is often the case.他沒有上學，這是常事。
As you will find out, all is now settled. 妳將會看到這樣的情況，壹切都已搞定了。
3、as引導的非限制性定語從句的位置比較靈活，既可位於所修飾的句子之後，也可位於句中或句首；而由 which引導的非限制性定語從句壹般不位於句首。例如：
As you will find out，I will never let you down.妳將會發現，我絕不會使妳失望的。
4、 as引導的定語從句只表示壹個眾所周知或意料之中的事，但which不受此限制。即as所引導的從句所表達的語意須和主句壹致
She has married again，which was unexpected.她又結婚了，真不可思議。這是出乎意料的(此處不可用 as)