这种 as 引导定语从句,主要见于 such ...as ...或 the same ...as ...中.
不妨先看这个句子：
They need people who can work under pressure.
这个定语从句很容易理解吧.
现在 people 前面加一个 such,定语从句就要用 as 引导：
They need such people as can work under pressure.
再如：
Don't make friends with such people as are lazy.
上述两句话中 as 在定语从句中作主语.此外它也可以是定语从句的宾语.如：
He used such expressions as he could find in the texts.
Here is such a big stone as no one can lift.
the same ...as ...也有这种用法.如：
She is wearing the same hat as you wore yesterday.
Do not make the same mistake as you did last time.
I'd like to read the same book as was read by Tom.
另外,“such + 名词”后面的定语从句只能用 as,而“the same + 名词”后面的定语从句除了 as 外还有 that.如：
Is this the same tool that you used yesterday?
两者的区别在于,the same ...as ...是指“同样的”,the same ...that ...是指“同一个”.