as的几种用法：1.做连词，固定搭配as……as，意思是“和……一样”；2.做连词，表示“当……的时候”；3.做连词，表示“因为”4.做连词，表示“尽管”等。
1.as做连词，固定搭配as……as，意思是“和……一样”
例句：

My sister is as tall as me.

我姐姐和我一样高。

He runs as fast as me/I. 

他和我跑得一样快。

2.as做连词，引导时间状语从句，意思是“当……的时候”

例句：

My mother was cooking fish as I arrived home.

当我到家的时候，我妈妈正在做鱼。

I was always hard up as a student. 

我当学生时总是很拮据。

3.as做连词，引导原因状语从句，意思是“因为”

例句：

As rain has fallen, the air is cooler. 

因为下过雨，所以空气比较凉爽。

4.as做连词，引导让步状语从句，意思是“虽然，尽管”

例句：

Young as he is, he has been one of the most famous painters in our country. 

虽然他还很年轻，但已经成为我们国家最有名的画家之一。

5.as做关系代词，引导定语从句

例句：

This is the same game as I played yesterday.

这和我昨天玩的游戏一样。

6.as做介词，意思是“作为”

例句：

French is known as the most beautiful language in the world.

法语以世界上最美丽的语言著称。

Personas are represented as individual people.

作为个体代表的人物角色。