1．时间状语从句：

as（当……的时候）, as soon as（一……就……）,

As / When I came home, I met an old schoolmate of mine.我回家的时候遇到了我的一位老同学。

I’ll ring you up as soon as I get an answer from him.我一有他的答复就给你打电话。

2．原因状语从句

表示原因的状语从句可以由as（由于）

As she was ill, she didn’t come to the party.由于病了，她没来参加晚会。

3．条件状语从句

so /as long as（只要）, as/ so far as（就……而言）,

As/So long as we don’t lose heart, we’ll find a way to overcome the difficulty.只要我们不灰心，我们就能找到克服困难的方法。

As/So far as I know, he is an expert on DNA.据我所知，他是一个DNA专家。

4．方式状语从句

方式状语从句常由as（与……一样）, as if（仿佛）, as though（仿佛）等词引导：

Do in Rome as the Romans do.入乡随俗。

Leave things as they are.让一切顺其自然。

She stood at the door as if (=as though) she were waiting for someone.她站在门口，仿佛在等人似的。（由as if或 as though引导的从句中可用虚拟语气）

5．比较状语从句

as（与……一样）

Our country is as big as the whole of Europe.我们的国家同整个欧洲一样大。

The result was not as/so good as I had expected.结果不如我预料的那么好。

6．让步状语从句

连词as也可以引导让步状语从句，但从句中要用特殊语序。如：

Young as he is, he knows a lot=Though he is young 或Young though he is, he….

他虽然年轻，但懂得很多。(though 也有这种用法，可以替换as,但although没有这种用法)

though还可以用作副词，放在句末。如：