3.在非限制性定语从句中，which和as引导定语从句时的区别：
a.which引导的从句不能放在主句前，而as引导的从句则既可以放在主句前，也可以放在主句后。
b.用能做关系代词，as和which都可以指代一个句子，但as有“正如”、“就像”之意，which则没有此含义。
He is a foreigner, as I know from his accent. (as指代一个句子，意为“正如”、“就像”)
He has to work on Sunday, which he doesn’t like. (which指代一个句子，没有“正如”、“就像”)
c.当从句的谓语动词是be或相当于联系动词的seem, become等时，可用as或which作主语。但当从句的谓语是行为动词时，只能用which作主语。
4.as 可用作连词，引导状语从句，构成省略形式“as + 过去分词”这一特殊结构，译成汉语时作用如定语。例如：
The bridge as shown in the picture is really wonderful. (as shown = as it is shown 可译为“图中的桥”)
See the answers as given at the end of this book. (as given = as it is shown 可译为“书后给出的答案”)
English as spoken is Australia is slightly different from British English.
(as spoken = as it is spoken 可译为“在澳大利亚讲得英语”)
请看下列的“as + 过去分词”结构用于修饰前面的名词：
The study as described in the article… 如文章中描述的那种学习
The program as conducted by… 由…指挥的（指导的）节目
The art as applied to medicine… 应用于医学的艺术
The products as seen on TV…电视上看到得产品
本文来源：http://www.010zaixian.com/yuwen/zaoju/349418.htm