在定语从句中，as 与 that 在用法上有什么区别？

看看这两个句子
This is the same bag as I lost yesterday.
This is the same bag that I lost yesterday.
在这两个句子中，as从句和that从句都属于定语从句的范畴，但as引导的定语从句表示它的内容与主句所讲的事物“相似”，即指同类的事物；that引导的定语从句表示它的内容与主句所讲的事物是“同一的”，即指同一事物。它们的前提都是在主句中有the same。所以这两个句子应该这样理解：

This is the same (= the same kind of) bag as I lost yesterday.

This is the same bag ( = the very bag) that I lost yesterday.

as 可以引导一个非限制性定语从句，as代指整个主句的内容并在定语从句中作主语、宾语或表语。这种as定语从句可放在主句的前边或后边，并有逗号和主句隔开。常见的由as引导的这类定语从句有：

as has been said before 如上所述

as may be imagined 正如可以想象出来的那样

as is well known 众所周知

as was expected 正如预料的那样

as has been already pointed out 正如已经指出的那样

as we all can see 正如我们都能看到的那样

但是THAT只能做关系代词引导限制性定语从句