一、as作连词的用法
1. as...as的用法
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。其基本结构为：as+ adj./ adv. +as。例如：
(1)This film is as interesting as that one.这部电影和那部电影一样有趣。
(2)Your pen writes as smoothly as mine.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。例如：
This dictionary is not as/so useful as you think.这本字典不如你想象的那样有用。
若有修饰成分，如twice, three times, half, a quarter等，则须置于第一个as之前。例如：
Your bag is twice as expensive as mine.你的袋子比我的贵一倍。
几个关于as...as的常见句型：
(1)as...as possible
Please answer my question as soon as possible.请尽快回答我的问题。
(2)as...as usual/before
She looks as pretty as before.她看起来和以前一样漂亮。
(3)as long as... (引导条件状语从句)
It took us as long as three years to carry out the plan.我们花了长达三年的时间才完成这项计划。
(4)as far as
He walked as far as the railway station yesterday evening.昨天傍晚，他一直散步到火车站。
(5)as well as
She cooks as well as her mother does.她烧菜烧得跟她母亲一样好。
一些带有as...as结构的常见短语归纳：
as busy as a bee像蜜蜂一样忙碌
as easy as ABC像ABC一样容易
as deep as a well像井一样深
as light as a feather像羽毛一样轻
as soft as butter像黄油一样软
as rich as a Jew像犹太人一样富裕
2. as用作连词引导时间状语从句
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：用when时，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;用while时，从句的动作为一过程，主句动作与从句动作同时进行或在从句动作过程中发生;用as时，主句和从句的动作同时发生，具有延续的含义。例如：
(1)It was snowing when he arrived at the construction site.他到达工地时，天正在下雪。
(2)He was watching TV while his mother was cooking.他母亲做饭时他在看电视。
(3) You will grow wiser as you grow older.你会随着年龄的增长而越来越聪明。
3. as用作连词引导原因状语从句
as，because，since都可以表示因果关系，连接原因状语从句，含义是"因为，由于"，但它们有区别：because表示的语气最强;as一般放在句首，语气较弱，较口语化;since常常用在书面语中，表示多为对方已知的、或稍加分析便可得知的原因，有时可译作"既然"。例如：
(1) He will succeed because he is in earnest.他一定会成功，因为他很认真。
(2) Since you are so sure of it he”ll believe you.既然你对此如此有把握，他会相信你的。
(3) As rain has fallen, the air is cooler.因为下过雨，空气比较清爽。