as 和 which 都可用作關係代詞來引導從句，它們兩個到底有什麼不同呢，這是同學們經常覺得困惑的地方。下面詳細講講它們之間的聯絡和區別，希望能解開大家心中的結。
一、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一點。
I live a long way from work，as/which you know．我離上班的地方很遠，你是知道這一點的。
The author was brought up in a small village, as/which is recounted in some of his stories. 作者是在一個小村莊里長大的，這個問題在他的一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……的一樣”的意思。例如：
She is a fine singer，as her mother used to be．她是一個很出色的歌唱家，和她母親當年一樣。
She is the same age as you (are). 她和你年齡一樣大。[與 連用]
I hope to get such a dictionary as he is using. 我希望有一本和你正在用的一樣的詞典。[與連用]
As water is to fish, so air is to man. 正如水對魚一樣重要，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. 正如你所知道的，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. 你將會看到這樣的情況，一切都已搞定了。
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一般不位於句首。例如：
As you will find out，I will never let you down．你將會發現，我絕不會使你失望的。
4、as所引導的從句所表達的語意須和主句一致，而 which則無此限制。例如：
She has married again，which was unexpected．她又結婚了，真不可思議。（此處不可用 as）