1. 用作介词，意为作为、当作、充当。如：
He treats me as a child. 他把我当小孩看待。
As your doctor, I advise you to eat less. 作为你的医生，我建议你少吃点。
2. 用作连词，注意以下用法：
(1) 表示随着。如：
As time passed, things seemed to get worse. 随着时间的推移，情况似乎变得更糟了。
若其后不接从句，而接名词，则用介词 with 表示随着。如：
With the development of modern agriculture and industry,
more and more waste is produced. 随着现代工农业的发展，污染越来越严重了。
(2) 表示让步(意为：虽然，尽管)，要用于倒装句(相当于 though, 但语气稍弱)。如：
Much as I like you, I couldn’t live with you. 我尽管很喜欢你，却不能和你一起生活。
Try as he would [might]，he couldn’t open the door. 他试过多次了，却仍打不开门。
注：倒装后位于句首的名词通常不用冠词。如：
Boy as he was, he behaved like a girl. 他虽然是个男孩，但举止却像个女孩。
3. 用作关系代词，主要用法有：
(1) 用在 such, same, as 等之后，引导限制性定语从句。如：
I live in the same place as Tom does. 我和汤姆住在同处。
Such people as knew him admired him. 认识他的人都钦佩他。
He smokes as expensive a cigarette as he can afford. 他抽一种他买得起的最贵的烟。
(2) 单独用作关系代词，引导非限制性定语从句，可放在主句之前(常译为：正如)或之后(常译为：这一点)，且主从句之间一般要用逗号隔开。如：
He was absent, as is often the case. 他缺席了，这是常有的事。
As was expected, he succeeded at last. 正如我们所料，他终于成功了。