as的语法特征总结
1. 用作连词，表示让步 (意为：虽然，尽管)，要用于倒装句，且倒装后坐落于句首的名词一般不需要冠词 (等于though, 但语气稍弱)：
Child as[though]he was, he did quite well. 他虽是个孩子，但已干得很好。
2. 用作关系代词，主要使用方法有2、
(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，这是常有些事。
As was expected, he succeeded at last. 正如大家所料，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了解。
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 (特别是文学体裁)中，as后可用倒装语序表示“…也一样”这种意思 (现代英语一般so表示这一使用方法)：
He travelled a great deal, as did most of his friends. 他去过很多地方旅游，他的多数朋友也是一样。
He plays the piano, as does his mother. 他会弹钢琴，他妈妈也会。
5. 用于 as if (from www.hxen.net)
as though, 意为“仿佛”、“好像”。两者一般可通用 (但注意不可以说 as although)。使用方法上注意什么时间：
(1) as if [though] 可引导状语从句和表语从句，从句谓语依据语义的需要，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. 天好像要下雨了。
It seems as if you’re right. 好像你是对的。
He talks as though he knew everything. 他夸夸其谈，仿佛无所不知。
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，它看着仿佛折断了似的(from www.hxen.net)。
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，好像遇见了一个难点。
She felt as if she lost something. 他感觉好像丢了什么东西。
(2) 当从句主语与主句主语一致，且从句谓语中又包含有动词be时，从句主语及其谓语中的动词be一般可以省去：
From time to time he turned round as though (he was) searching for someone. 他不时地四下张望，仿佛在找人。
He opened his mouth as if (he was)to speak. 他张开口，仿佛要说话似的。
