as可以作连词、介词及关系代词。现将其用法小结如下:
一、 as作连词的用法:
1。 作当……的时候,引导时间状语从句。注意与when、 while的用法区别。
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。
   
When the clock struck twelve, all the lights in the street went out。  
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。如:
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……。如:
The girl dances as she sings on the stage。
    
He looked behind from time to time as he went forward。 
B)表示两个同步发展的动作或行为,译为随着……。如:
As time went on / by, she became more and more worried。
    
As children get older, they become more and more interested in the things around them。  
C)表示两个短促行为或事件几乎同时发生。如:
I thought of it just as you opened your mouth。
    
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。如:
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。如:
As / Since you're not feeling well, you may stay at home。 
As he wasn't ready in time, we went without him。
   
3。 as=in the way that, 作像、按照……的方式解,引导方式状语从句。如:
He speaks English as Americans do。
Remember, you must do everything as I do。
   
4。 用于as。。。as。。。或not so/ as。。。as。。。中,前一个as是副词,后一个as是连词,引导比较状语从句。如:
They helped the old man as often as possible。  
I don't speak English so/ as well as he does。
    
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。如:
Tired as they were, they walked on。
Child as he is, he knows a lot。
   (child前不用冠词)
二、 as作关系代词,引导定语从句,作正如、这一点解。如:
He is very careful, as his work shows。 
As is well-known, Taiwan belongs to China。
   
as的这种用法,请看2001年的高考题:
        is known to everybody, the moon travels round the earth once every month。
A。 It    B。 As    C。
   That   D。 What
 (字母下划横线的为正确答案)
另外,当先行词被the same, such等词修饰时,常用as来引导定语从句。如:
Such a clever boy as he can learn anything quickly。
    
I was reading the same book as he bought yesterday。  
。