as 是英语当中非常重要的词汇，应用非常广泛，现在总结如下：
as可以充当4种功能：连词，定语从句的引导词，介词和副词。
一、as作连词
1.as...as “和……一样”，
表示同级的比较。第一个as为副词，第二个as为连词。其基本结构为：as adj./ adv. as。例如：
(1)This film is as interesting as that one.
(2)Your pen writes as smoothly as mine.
否定式为not as/so adj./ adv. as。例如：
This dictionary is not as/so useful as you think.
几个关于as...as的常见句型
as long as... (也可引导条件状语从句) 只要-----
It took us as long as three years to carry out the plan.
as well as 和----
She cooks as well as her mother does
2. as用作连词引导时间状语从句
这里表达相同汉语含义的还有while, when 等等，不一一比较了
二、as作关系代词的用法
1. 限定性定语从句
①用于the same...as结构中
This is the same book as I read last week.
②用于such...as结构中
I don’t like such books as he recommends.
③用于"so adj. a/an n. (单数) as "结构中
I am not so strong a man as I was.我已经没有从前那么强壮了。
2. as单独引导非限定定语从句时
(1) She is late, as is often the case.
(2) To shut your eyes to facts, as many of you do, is foolish.
三、as作介词的用法
as作介词，意思是"作为"，"以……身份"。例如：
He came to China as a tourist five years ago.
四、as作副词的用法
to the same degree or amount; equally "相同地"，"同样地"。例如：
They don’t have as many airplanes.他们没有同样多的飞机。
*As和 like 都可以做介词,但词义是不同的,
As 作为, 以…资格,
Like 像….与…相似
As a scientist, he dedicated to the truth.
He wants me to go with him as his secretary.
She works like a beaver. 她工作勤勤恳恳
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