as在英语中是最活跃的词之一，它既可作副词、连词、介词，也可作关系代词。下面就as引导的各类从句的用法，试作归纳，供学习时参考。
一、表示比较，意为“象……一样，正如，如同”
She’s very tall, as is her mother. 她个子很高，和她母亲一样。
二、表示方式、程度、情况。意即“照……方式、如同、象”

They did as I had asked.  他们是按照我的要求做的。
三、表示时间，意为“当……时候”，“一边……一边”“随着……”
As he grow older he gained in confidence.  随着年龄的增长他的信心增强了。
四、表示原因，意为“因为，由于”
As you were out, I left a message.   你不在，所以我留下了一张字条。
五、表示让步，意为“尽管；虽然；即使”（表语一般放在本从句之首；如有可数名词单数，前面也不要冠词）
Child as he is ,he can tell black from white. 尽管他是个孩子，但他能够分辨黑白。
Happy as they were, there was something missing. 尽管他们很快乐，但总缺少点什么。
六、引导定语从句
1）as引导非限制性定语从句，常指主句所表达的整个事情，可置句首、句中、句末，常见 as we all know,as I see it 照我看来，as it appears to me 依我看来。
As we all know, oceans cover more than 70 percent of the earth.
正如我们所知道的，海洋占地球面积70％以上。
2）as引导限制性定语从句
From every corner of the island little people came to see the "Man Mountain~as they called Gulliver. 小人们从海岛的各个角落来看他们叫做‘人山’的格利佛。
3）as与 such, so, the same 连用，此时先行词是一个具体的名词，as在从句中可作主语、宾语或表语。
The book is written in such easy English as beginners can understand. 这本书是用初学者也能读懂的浅易英文写的。
She studies in the same department as I do. 他和我在同一个系统学习。

Here is so difficult a problem as none of us can solve.

这是一个我们都不能解决的难题。