"As”可以用作关系代词引导定语从句。
一：as可以代替整个句子或一件事，引导非限制性定语从句，在从句中作主语、宾语或表语，意为“正如……，就像……，据……”；从句位置可置于主句之前或之后，有时也可作为插入语置于句中，一般用逗号和主句隔开。
as的这种用法常见于以下这些结构:as we can see, as can be seen, as we know, as is （well） known,as may be imagined,as often happens,as has been said before,as was mentioned above,as is often the case,as/indeed it is,etc.
多看一些例句:As is well known，great changes have taken place in China.or:Great changes,as is well known,have taken place in China.众所周知，中国发生了巨大的变化。( as在定语从句中作主语。)
This experiment is very simple，as indeed it is.实验非常简单，事实上也确实如此。(as在定语从句中作表语。)
注意：① which在引导非限制性定语从句时，也有和as一样的功能，可以代替整个句子，但which只能用在主句之后。
②在as所引导的非限制性定语从句中，如果运用的是被动语态结构，be动词可以省略；但which没有此用法，be动词不可省。例如:She married him，which was unexpected.＝She married him as（was） unexpected.她嫁给了她，这一点大家都没有料到。（作主语，代替前面的句子；用which时was不可省。）
二 as除了代替整个句子，也可以代替主句中一个特定的词。as在它引导的定语从句（限制性或非限制性）中可以充当主语，宾语和表语；在这种用法中它经常和such, so, the same 等词搭配使用；且当先行词中有such, so, the same等词修饰时，关系代词通常必须用as。
例如:1. They were satisfied with this decision，as was agreed beforehand.他们对这个决定都满意，这事先已经经过他们同意了。( as在定语从句中作主语，其先行词是this decision)
2. I never heard such stories as he tells.我从未听过他讲的这类故事。( as在定语从句中作宾语，先行词是such stories.)
3.The place doesn't look the same as it was before liberation.这个地方看上一去和解放前大不一样了。( as在定语从句中作表语，先行词是same)
4.Have you bought the same book as I referred to yesterday?你有没有买我昨天说的那本书啊？( as在定语从句中作宾语，先行词是book)