首先明白引导定语从句的as属于关系代词，在定语从句中作名词成分。
as引导非限定性定语从句与which的区别是，（1）as翻译成“正如，正像” which翻译成“这，那=and this/that” （2） as引导的定语从句可以放句中，句首，句末。
As we realized,English is important for us.
Air,as we know,is a gas.
He was a teacher,as can be seen.
She has married again,which was unexpected.
常见的有as we all know,as has been said above,as has been pointed out等
as引导限定性定语从句必须与前面the same,so,such,as等构成一个结构。
There are as many dictionaries as are needed. 所需要的词典都有了
He is such a good teacher as we all like. (定语从句)
对比：
He is such a good teacher that we all like him. (结果状语从句)
另外注意the same…that表示相同，thesame…as表相似
I have the same pen as you have.
This is the same pen that I lost yesterday.
I have never heard such stories as he tells.
He lifted so heavy a stone as no one else can lift.
有时通用
The meeting was put off,as/which was exactly what we want.
He was a doctor,as/which I knew from his manner.
