as在限制性定语从句中的以用法及as与that的区别。

as 可以用作关系代词，在限制性定语从句中 ，多同such 或the same 连用，构成 such......as
和 the same...as结构 。as 的先行词可以是人或物。如
he is not such a fool as he looks.他貌似憨傻 实则不然。
the same ...as 表示同一类的 如 this is the same book as i bought yesterday.
the same...that 表示同一个 如 this is the same book that i am looking for.
不过 有时区别不大 可以互换
如 we drove out of the town by the same road as/that we had entered by.
我们从进城的原路开车出城去。

当先行词被such 修饰时，定语从句需用as 引导，
如： It is such an easy question as every one can work it out .
当先行词被the same 修饰时，用as 或that 都可以，但意思不同。the same ...as (指同样或同类的）；the same ....that (指同一个）。如：I have the same book as you have 。 This is the same watch that I lost .

as引导定语从句，其实不是专指限定性定语从句。而是一种特殊的定语从句。
as做关系代词，主要分3种：
1：构成such...as的结构。可以看做是固定表达，所以不能用that替换as。
e.g. We have such apples as you never saw.我们有你从未见过的苹果.
2：such 和as连在一起，也可以看做是固定表达，不能替换that。
e.g. There was a look of fear in his eys,such as people have when they are suddenly awakened.
他眼中透露出一种人们突然被叫醒时，露出的惊恐。
3：as可以单独引导定语从句，指代整个句子。
e.g.He was a foreigner, as I knew from his accent.
从他的口音中，可以听出是个外国人。as指代He was a foreigner的整个句子。

关于that：
在限定性定语从句中，作为从句的先行词，是不能用逗号和主句隔开的。
另外，that指代的名词，在主句中通常做主语或者宾语。这时可以和which替换。
e.g. These are the books which/that I ordered.

如果有介词提前，也就是从句的介词放到先行词的前面，只能用which不能用that。