as在定语从句中是十分常见的，那么as引导定语从句的用法又是什么呢?
as作关系代词时，引导定语从句在从句中充当主语、宾语、介词宾语或表语，既可以指人，也可以指物。
(1)as引导定语从句时，有如下结构：“the same …as”，“such …as”， “as…as”“so many (much, few, little)…as”，译作“像……一样的人(或物)”。例如：
This is such an easy novel as you can read.
这本小说很容易，你能读得懂。
They have never read so many books as I read last year.
他们从来都没有读过像我去年读过的那些书。
(2)as单独引导定语从句时，修饰整个主句或主句中的一部分，可以放在句首、句中或句末。
当as在从句中作主语时，从句的谓语通常是被动语态或系表结构;as在从句中作宾语时，从句的'谓语常用“情态动词+动词原形”构成。常见的结构如下：
as is known to all 众所周知
as has been said above 如上所述
as was expected 正如预料的那样
as has been pointed out 正如指出的那样
as we all can see (know, hear)正如大家看到的(知道的、听到的)那样。例如：
As is known to everybody, the moon travels round the earth once every month.
众所周知，月球每个月绕地球转一周。
