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一、as作为介词的用法：
1.表示“作为”，“以……身份”。例如：Tom found a job as a teacher.
2.表示“像…一样”，例如：They were all dressed as policeman.
二、as作为副词的用法：
1.表示“与…等同”。如：They don’t have as many boxes.
2.表示“像，如”。如：As before, he remained slient.
三、as可作从属连词，作关系副词时可以引导比较、时间、原因及方式等四种状语从句，作关系代词时可以引导定语从句：
(一)as作从属连词引导状语从句时：
1.表示“在…期间，当…的时候”引导时间状语从句，要注意其与when、while的用法区别。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。
下列情形时,只用as，而不用when或while。
1)表示两个同步发生的动作或行为，意思是“随着…的发展”。如：
As time went on, she became more and more worried.
As we get older, we become more and more interested in everything.
2)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 sings as she goes to supermarket.
He looked behind occasionally as he went forward in the forest.
3)接在名词后面表示某一个年龄段时。如：
As a young man, he was active in answering questions.
4)表示两个短暂行为或事情几乎同时发生。如：
I watched her as she read the book.
I thought of it just as you opened your mouth.
when作“当……的时候”解,可以指较短的时间，也可指一段时间。如：
It was raining when he arrived at the factory.
When I was at college, I could speak several foreign languages.
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：He was watching TV while his mother was cooking.
2.as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。与because，since相比，as一般放在句首，语气较弱，较口语化。如：

As you’re not feeling well, you may stay at home.
As he wasn’t ready, we went without him.
3.as作“像”、“按照……的方式”解，引导方式状语从句。如：
Leave things as they are.
Remember, you must do everything as I do.
4.表示“虽然，尽管”，引导让步状语从句，多在主句之前，且常用倒装语序，模式为：adj. /adv. /n. /v. as 主语 谓语 主句。如：
Young as he is, he knows much about electricity.
Try as he did, he failed again.
知识拓展：用于as…as或not / so/ as…as中，前一个as是副词，后一个as是连词，引导比较状语从句。如：
They help the old as much as they can.
I don’t speak English as well as she does.
(二)as作从属连词引导定语从句时，表示“正如”：
1.引导性定语从句。如：
He isn’t the same person as he used to be.
I have never heard such story as he is telling.
I am not so strong a man as I was.
2.引导非性定语从句，修饰整个主句或主句的一部分，可在句首、句中、句末。如：He is very careful, as we all know.
As is well known, she has two sisters.
This machine, as might be expected, has stopped operating.
总之，通过以上的分析，我们对as的词性和用法有了比较全面的理解和把握，这样才能在实际的运用中做到心中有数，应付自如。