一、作副词，意为“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。
二、作连词，
1. 引导时间状语从句
as与when，while都是引导时间状语从句的从属连词，含义都是"当……的时候"。但它们有区别:
(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。从句的动作可以与主句的动作同时发生，也可以先于主句的动作发生。例如:
John was having his dinner when I saw him. 当我看到约翰的时候，他正在吃饭。
She can write only when the baby is asleep. 只有婴儿睡着的时候，她才能写作。
(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或在从句动作过程中发生。例如:
We must strike while the iron is hot. 要趁热打铁。
While we were reading, the teacher came in. 我们正在读书的时候，老师走了进来。
(3). 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……”。例如:
The girl dances as she sings on the stage.那个女孩在舞台边唱歌边跳舞。
He looked behind from time to time as he went forward. 当他朝前走时，不时地向后看。
② 表示两个同步发展的动作或行为,译为“随着……”。例如:
As time went on / by, she became more and more worried. 随着时间的流逝，她变得越来越焦虑。
As he grew older, he became more intelligent. 随着他年龄的增长，他变得更有才智了。
③ 表示两个短促行为或事件几乎同时发生。例如:
I thought of it just as you opened your mouth. 恰好在你开口时，我想到了它。
Just as the flying worm hit her face, she gave a loud cry. 恰巧在飞虫撞到她脸上时，她大哭起来。