as可以用作介词、副词和连词。
I. as 作介词，意思是 “作为”,“以…身份”，“如同”
1) Alfred worked as a dustman for the Ellesmere Corporation. 阿尔弗雷德在Ellesmere公司当清洁工。
2) People who work in offices are frequently referred to as “white-collar” workers. 坐办公室的人常常被称作”白领工人”。
II. as作连词的用法
1. as…as, 意为 ”和…一样”, 用于形容词或副词的比较，as + adj/adv + as…, 第一个as为副词，第二个as为连词。
1) In 1991, there were twice as many detached houses as semi-detached. 1991年独立式住宅房的数量是半独立住宅房的两倍。
2) The App. is not as useful as we expected. 那个App不像我们期待的那么有用。
3) The girls performed as well in geography as the boys in 2010. 2010年女孩在地理课的考试成绩和男孩一样好。
4) He will be earning only half as much as he used to. 他将来挣的钱只有他现在的一半。
2. 引导时间状语从句，“随着”, “当…时”，和when, while用法相同，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) When she was young, she lived with her grandparents. 她小时候和祖父母住在一起。
2) I was preparing our dinner while you were sleeping. 你睡觉的时候我在准备晚餐。
3) You will grow wiser as you grow older. 随着年龄增长你会变的越来越聪明。
3. 引导原因状语从句，“由于”, “因为”, 和because, since用法相同，但他们有区别：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) As there were not enough lifeboats for everybody, 1500 lives were lost. 由于没有足够的救生艇运载所有乘客，结果1500人丧生。
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
4. 引导让步状语从句，”尽管”, “虽然”, 句子结构稍有不同，例如：
1) Old as he is, he still jogs every day. 他尽管上了年纪，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，却没人受伤。
3）Hard as he worked, he still failed to make both ends meet. 他尽管努力工作，仍未能做到收支平衡。
III. as作关系代词，引导定语从句
1. as引导定语从句与其他词连用
①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.
②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.
③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。(修饰的是整个主句)
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)
3) As you know, I like music
