as作介词的使用
as作为介词时，可以表示“像……”，这时它可以和like互用；as还可以表示“以……身份”或“作为……”，“当作……”，但like不能。
As a philosopher, Emerson was dedicated to the truth.
作为一名哲学家，爱默生致力于追求真理。
They talked as old friends.
他们谈起话来像是老朋友。
可替换为They talked like old friends.
as作连词的使用
作“当……的时候”,引导时间状语从句。注意与when、 while 的用法区别。
A. when作“当……的时候”解,可以指较短的(一点)时间,也可指一段时间。
When he was at college, he could speak several foreign languages.
当他上大学期间，会说很多门外语。
When the clock struck twelve, all the lights in the street went out.
当晚上12点钟一到，街上的灯便熄灭了。
B. while常表示一段较长的时间或一个过程,强调主句谓语动词与从句谓语动词同时发生。
Play while you play. Work while you work.
玩儿要尽情；工作要投入。
C. 但属下列情形时,只用as, 而不用when或while。
1)用于表示同一个人的两种动作交替进行,指“一边……,一边……”。
The girl dances as she sings on the stage.
姑娘在舞台上边跳边唱。
He looked behind from time to time as he went forward.
他一边往前走，一边往回望着。
2)表示两个同步发展的动作或行为，译为“随着……”。
As time went on / by, she became more and more worried.
随着时间的推移，她越发焦虑起来。
As children get older,they become more and more interested in the things around them.
随着孩子越来越大，他们对身边的事物也越发感兴趣了。
3)表示两个短促行为或事件几乎同时发生，一触即发。
I thought of it just as you opened your mouth.
你刚一张嘴，我就知道你要说什么。
4) 后接名词表示某一年龄段时,用as。
As a young man, he was active in sports.
在他年轻时，他热衷于体育运动。
as作副词的使用
1．表示同等程度、质量、数量等，作“一样地……”解释。
Steve runs fast, but I run just as fast.
史蒂夫跑的很快，但是我跑的和他一样快。
Tom has a lot of money, but I don't have as much.
汤姆有很多的钱，而我的钱没有他（一样）多。
2．用在形容词或者-ed形式的过去分词前，相当于形容词或过去分词作定语，但as仍然保留其原来的词汇意义。
In the dead of night I was suddenly wakened by a cry as made by a wolf.
在寂静的黑夜里，我突然听到了一声如同狼嚎一样的叫声。
The old man began to describe the accident as seen through the eyes of a witness.
这位老人开始描述这一起事故，如同是一个证人亲眼目睹了一般。