As作为一个多意多词性的单词在初中课本出现的频率较高，很多同学对它的用法模糊不清，现将这个看似简单用法多变的as归纳如下：
A. adv.
1.相同、 相似
Mary runs fast， but I run just as fast.
玛丽跑得快，但我也跑得快
2.比如
All kinds of animals， such as birds， cats and gods.
各种动物，例如鸟、猫和狗。
3.被看作、被认为是（常用在形容词和分词之前）
man as described by scientist科学家所描述的'人类
man as different from the other animals有别于其他动物的人类
B. 同样、像 conj.
1. as....as......中间用形容词和副词原型
She runs as fast as me.她和我跑得一样的快
not so...as= not as...as
She isnt as pretty as her sister. 她没有她姐姐漂亮。
2.uch ......as....
Such animals as dogs and pigs. 诸如猪狗类的动物。
3.so....as与so...that的区别
Here is so a heavy a box as no man can lift.（同义句）
Here is so a heavy a box that no man can lift.这儿的如此重的一个箱子，没有人能搬动它。
4.such.....as 与 such......that......的区别
Here is such a heavy a box as no man can lift.（同义句）
Here is such a heavy a box that no man can lift.这儿的如此重的一个箱子，没有人能搬动它。
以上四个句子意思相同，用as则不用it;用that便要用it，因为as为关系代词，that为连接词。