一、as用作关系代词引导定语从句
1. as 引导非限制性定语从句，一般不指代某个具体的名词或代词，而是代表整个主句或主句的一部分。这种从句可放在主句之前，也可以放在主句之后，有时还可以放在主句的中间。如：As is reported in the newspapers, talks between the two countries are making progress. 正如在报纸上所报道的，两国间的谈判有所进展。Grammar is not a set of dead rules, as has been said above. 如上所说，语法不是一套死条文。Spiders are not insects, as many people think, nor even nearly related to them.蜘蛛不是昆虫，尽管许多人都这样认为，它甚至与昆虫几乎没有联系。2. as 引导限制性定语从句，主要用于the same…as（与……相同），such…as（像……一样），as…as（与……一样）。如：（与She knew he felt just the same as she did. 她知道他的感受和她一样。Such a man with a good temper as Mr. Li is is easy to get along with. 像李先生这样好脾气的人是容易相处的。He smokes as expensive a cigarette as he can afford. 他抽一种他买得起的最贵的烟。
二、as作为从属连词引导时间状语从句
表示“当……的时候”或“>一边……一边”，强调主句的动作和从句的动作同时发生。如：I saw him as he was getting off the bus. 当他下公共汽车的时候我看见了他。The boy was singing a popular song as he was bicycling. 这男孩边骑车边唱歌。As she grew older, she became less active. 当她长大一点的时候，就变得不那么活泼了。As she grew older, she became less active.
三、as作为从属连词引导原因状语从句
as 引导的原因状语从句常放在主句前，表示“因为，由于”。如：As she is a model teacher, she gives a good example to others. 因为她是一名模范教师，她给其他人树立了一个好的典范。As he wasn’t ready in time, we went to the cinema without him. 由于他没有及时准备好，我们就去看电影了，没有等他。
三、as作为从属连词，引导让步状语从句
as引导让步状语从句比较特别，它要求将主语和谓语或谓语的一部分倒装。如：Old as he is, he is still full of energy. 尽管年纪大了，但他依然精力充沛。Try as she may, she never succeeds. 尽管她很努力，但总是不成功。Try as she may, she never succeeds. Much as I admire Mr. Wang as a poet, I don’t like him as a man. 尽管我喜欢王先生的诗歌，但我不喜欢他的为人。注：当倒装后置于句首的名词为单数可数名词时，习惯上不带冠词。如：Boy as he was, he was made king. 尽管他还是个孩子，却被立为国王了。
四、与其他词搭配使用构成从属连词引导从句
1. as if / as though。表示 “好像，仿佛”，引导表语从句或方式状语从句。如：It looks as if / as though it is going to snow. 看起来要下雪啦。He looks as if / as though he had seen a ghost. 他的脸色看起来好像他看见了鬼。He talks as if /as though he knew everything about it. 他说话的口气好像他已经全部都知道了。< it. about everything>
2. as [so] long as。“只要”，引导条件状语从句。如：< it. about everything>As [So] long as you can keep away from them, you will be safe. 只要你能远离他们，你就安全了。I’m happy as long as you are happy. 只要你幸福，我就感到幸福了。I’m happy as long as you are happy.
3. as [so] far as。表示“在……的范围内”，引导限制状语从句。多用于下列结构：as [so] far as I know（据我所知），as [so] far as I can see, as [so] far as I’m concerned（依我看）。如：（依我看）。如：As [So] far as I know, the couple have been abroad for about five years. 据我所知，这对夫妇已经约五年了。There’s only one thing to be done now, as far as I can see. 依我看。现在只有一件事情要做。
4. as soon as。表示“一……就”，引导时间状语从句。如：As soon as he got off the plane, he telephoned her girl friend. 他一下飞机就给他的女朋友打了个电话。He started as soon as he received the news. 他一得到消息就出发了。
总之，由于as的用法复杂，同学们在学习时一定要注意比较其在不同句子中的作用，判断其连接的是什么样的从句，多做一些关于相关练习，这样便可掌握其用法了。
本文来自：逍遥右脑记忆 http://www.jiyifa.net/gaozhong/173479.html
相关阅读：2012年高考英语语法练习：情态动词和虚拟语气