1. 用作连词，表示让步 (意为：虽然，尽管)，要用于倒装句，且倒装后位于句首的名词通常不用冠词 (等于though, 但语气稍弱)：
Child as[though]he was, he did quite well. 他虽是个孩子，但已干得很不错。
2. 用作关系代词，主要用法有二：
(1) 用在such, same, as等之后，引导限制性定语从句：
This is the same watch as I lost. 这块表跟我丢失的那块一样。
Such men as (=Those men who) heard of him praised him. 听说过他的人都赞扬他。
(2) 单独用作关系代词，引导非限制性定语从句，可放在主句之前 (常译为：正如)或之后 (常译为：这一点)，且主从句之间一般要用逗号隔开：
He was absent, as is often the case. 他缺席了，这是常有的事。
As was expected, he succeeded at last. 正如我们所料，他终于成功了。
3. 用来表示目的，下面两种句子结构都可以：
仔细写以便把每句话都写清楚。
正：Write carefully so as to make every sentence clear.
正：Write so carefully as to make every sentence clear.
4. 在正式文体 (尤其是文学体裁)中，as后可用倒装语序表示“…也一样”这类意思 (现代英语通常so表示这一用法)：
He travelled a great deal, as did most of his friends. 他去过许多地方旅游，他的多数朋友也是一样。
He plays the piano, as does his mother. 他会弹钢琴，他母亲也会。
5. 用于 as if (from www.yygrammar.com)
as though, 意为“仿佛”、“似乎”。两者一般可通用 (但注意不能说 as although)。用法上注意几点：
(1) as if [though] 可引导状语从句和表语从句，从句谓语根据语义的要求，可用陈述语气 (若可能为事实)或虚拟语气 (若为假设或不太可能是事实)：
It looks as if it is going to rain. 天似乎要下雨了。
It seems as if you’re right. 似乎你是对的。
He talks as though he knew everything. 他夸夸其谈，好像无所不知。
When a pencil is partly in a glass of water, it looks as if it were broken. 当把一支铅笔的一部分放在一杯水里时，它看起来好像折断了似的(from www.yygrammar.com)。
当主句谓语是过去时态时，从句谓语常可用陈述语气：
He paused as though he found a difficulty. 他停了下来，似乎遇到了一个难题。
She felt as if she lost something. 他觉得似乎丢了什么东西。
(2) 当从句主语与主句主语一致，且从句谓语中又包括有动词be时，从句主语及其谓语中的动词be通常可以省去：
From time to time he turned round as though (he was) searching for someone. 他不时地四下张望，好像在找人。
He opened his mouth as if (he was)to speak. 他张开口，好像要说话似的。
6. 关于 as…as, 用法注意(from www.yygrammar.com)：
(1) 中间只能用形容词或副词的原级，不能用比较级或最高级。如果在第二个as之后使用独立的人称代词，一般说来用主格 (较正式)或宾格 (非正式)均可：
He’s as old as she her. 他年纪跟她一样大。
注意：要是第二个as之后的人称代词跟有谓语动词，则该人称代词只能用主格：
He is as old as she is.
但是有时用主格或宾格含义差别很大：
You love him as much as I (=as I love him). 你爱他像我爱他一样。
You love him as much as me (=as you love me). 你爱他像爱我一样。
(2) 在肯定句中用as…as, 在否定句中用not as…as或not so…as 均可：
He doesn’t study as [so] hard as his brother. 他学习不如他弟弟努力。
(3) 要注意比较对象的一致性，即比较的双方必须是可以比较的同类人或物，否则会出错：
正：The population of China is much larger than that of Japan. 中国人口比日本人口多得多。
误：The population of China is much larger than Japan.
(此句错在将 the population of China 与 Japan 来比较，比较对象不一致)
(4) 如果涉及数量或程度，可用以下两个结构：
①as much＋ (不可数名词＋) as
②as many＋ (可数名词复数＋) as
You must speak English as much as possible. 你必须尽可能地多说英语。
You’ve made as many mistakes as I have. 你犯的错误和我犯的一样多。
(5) 请注意下列结构的词序：
她和她妈妈一样是位好厨师。
正：She is a cook as good as her mother.
正：She is as good a cook as her mother.
误：She is as a good cook as her mother. (from www.yygrammar.com)
(6) 修饰as…as结构的常见词语有nearly, almost, just, exactly, half, quite, twice, three times等，请注意这些修饰语都只能放在as…as 结构之前 (而不能置于其中或其后：
This rope is twice as long as that one. 这根绳子比那根长一倍。
(7) as…as结构的下列用法，属于“异质比较” (注：
主、从句的主语和谓语一致时，从句的主语和谓语有时可以省略)：
The room is as long as (it is) wide. 这房间的长宽尺寸一样。
The news was as unexpected as (it was) welcome. 这消息来得突然，但受人欢迎。
She is as clever as (she is) beautiful. 她既聪明又漂亮。
One is as light as the other is heavy. 一个轻，另一个重。
(8) as…as结构在一定的上下文中可以用其省略结构 (即省去其中的一个 as)：
She’s clever, but her brother is just as clever. 她很聪明，但她兄弟也一样聪明。
The woman married a man poor as herself. 这个女人嫁给了一个与她一样穷的男人。