引导状语从句的连词as可以用作关系代词,引导定语从句,因而被称作准关系代词.
1.准关系代词as引导限制定语从句,用于以下3个结构中,as在从句中作主语、宾语或表语：
a.so / such…as
Such people as were mentioned by him were honest.(as作主语)
I’ve never seen such a clever man as he is.(as作表语)
Last term our maths teacher set so difficult an (= such a difficult) examination problem as none of us worked out.
b.the same…as
Modern statemen are often faced with the same problem as defeated the ancient Romans.(as 作宾语)
I like the same book as you do.(as 作宾语)
c.as…as
As many children as came were given some cakes.(as 作主语)
He is as diligent a man as ever lived.(as 作主语)
补充：
a.such…as 与 such…that的区别
such…as 引导定语从句；such…that引导结果状语从句.
They talked in such simple English as children could understand.
他们用孩子们能听懂的简单的英语交谈.
They talked in such simple English that children could understand it.
他们用十分简单的英语交谈,连孩子们都能听懂.
I will provide you with such things as you may need.
我要供给你一些你可以用得着的东西.
He shut the window with such a force that the glass broke.
他如此用力地关门,结果玻璃碎了.
b.the same…as与the same…that的区别：
the same…as指两物相似,the same..描述的是同一物.
This is the same bag as I lost yesterday.
这个书包和我昨天丢失的（书包）相似.（相似物）
This is the same bag that I lost yesterday.
这正是我昨天丢失的那个书包.（同一物）