as连词用法
1、作"当……的时候",引导时间状语从句。
① when作"当……的时候"解,可以指较短的(一点)时间，也可指一段时间。如:
When he was at college, he could speak several foreign languages. （表示一段时间）
When the clock struck twelve, all the lights in the street went out. （表示时刻）
② 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。
Work while you work. Play while you play.
③ 但属下列情形时,只用as，而不用when或while。
A) 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.
B)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 wor
ried.
C)表示两个短促行为或事件几乎同时发生。
Just as the flying worm hit her face, she gave a loud cry.
D) 后接名词表示某一年龄段时，用as。
As a young man, he was active in sports.
2、as=since, 作"既然"、"由于"解，引导原因状语从句，常用来表示已为人们所知或显而易见的原因或理由。
As / Since you're not feeling well, you may stay at home.
3、as=in the way that，作"像"、"按照……的方式"解，引导方式状语从句。
He speaks English as Americans do.
4、用于as...as...或not so/ as...as...中,前一个as是副词（副词后面跟形容词、动词、副词）,后一个as是连词（连词后面可以跟动词、形容词、句子）,引导比较状语从句。
They helped the old man as often as possible.
5、作"虽然"、"尽管"解，引导让步状语从句。常用倒装语序,即adj. /adv. / n.+ as +主语+谓语+主句。
Tired as they were, they walked on.
