as可作介词、副词、连词和名词四种词性。中文含义有“像”、“作为”、“像……一样”、“当……时”、“随着”、“因为”、“照……方式”、“阿斯”，还可以指“A”的复数。
as…as…意为”和……一样”，表示同级的比较。使用时要注意第一个as为副词，第二个as为连词。其基本用法为as+ adj./ adv. +as…。例如：
This film is as interesting as that one (is).
这部电影和那部电影一样有趣。
Your pen writes as smoothly as mine (does).
你的钢笔书写起来和我的一样流畅。
其否定式为not as/so +adj./ adv. +as…结构。例如：
This dictionary is not as/so useful as you think it is.
这部字典不如你想象的那样有用。
I didn”t catch as many as I had expected.
我抓到的不如我预想的那么多。
若有修饰成分，如twice, three times, half, a quarter等，则其必须置于第一个as之前，例如：
You are not half as clever as you think you are.
你可不象自己想象的那么聪明。
She isn’t going out with a man who is twice as old as she.
她不打算和一个比她大一倍的人出去散步。