as一．as 用作连词的用法：
1. 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。
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.
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.
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动作几乎同时发生。如：
i thought of it just as you opened your mouth.
just as the flying worm hit her face, she g*e 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。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。
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 dangerour place.
as/since he wasn't ready in time, we went without him.
3. as作连词引导让步状语从句
as与although (或though)，however (或no matter how)等都可以引导让步状语从句，含义是"虽然，尽管"，但它们有区别：although语气稍正式些，可放在句首，也可放在句中，主句中不能再用but，但可以用yet；as所表示的语气较强，引导的让步状语从句用倒装语序，即adj. /adv.
/n.+ as +主语+谓语+主句；however引导让步状语从句时，它的后面可跟形容词或副词，也要用倒装语序。例如：
1) although he is quite old, he still jogs every day.他虽然年纪相当大，仍然每天慢跑。
2) strange as it may seem, nobody was injured in the accident.这次意外虽然显得令人不可思议，却没有人受伤。
tired as they were, they walked on.
child as he is, he knows a lot. (child前不用冠词)
3) however hard he may try, he will not attain his goal.无论他怎样努力也达不到目标。
作连词引导方式状语从句。
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.
5. 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。
用于as...as...或not so/ as...as...中，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.
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
二、as作关系代词的用法
关系代词as引导定语从句时，既可以单独使用，也可以与其他词连用，其用法要比that和which更为复杂。
1. as引导定语从句与其他词连用
用于the same...as结构中
this is the same book as i read last week.这本书和我上周读的那本是一样的。
i was reading the same book as he bought yesterday.
用于such...as结构中
i don’t like such books as he recommends.我不喜欢他推荐的那些书。
such a clever boy as he can learn anything quickly.
用于"so +adj. +a/an + n. (单数) +as "结构中
i am not so strong a man as i was.我已经没有从前那么强壮了。
2. as单独引导定语从句
as单独引导定语从句时，先行词可以是一个词，也可以是一个句子或短语。例如：
1) she is late, as is often the case.她迟到了，这是经常的事。(先行词是整个主句)
2) to shut your eyes to facts, as many of you do, is foolish.对事实视而不见---你们好多人都是如此的愚蠢。(先行词是不定式短语)
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