一、as用作介词：

第一种同like，意思是“像，如同”，例如：

The bomb was disguised as a package. 炸弹伪装成一个包裹

The news came as a shock. 消息传来，令人震惊。

第二种是“作为……身份”，这个时候与like就有区别了，例如：

She works as acourier. 她的职业是导游。

She works like acourier. 她像导游一样工作。

二、as用作连词：

第一种是引导状语从句，可表时间、原因、让步、方式等，例如：

(a) As she grew older she gained in confidence. 随着年龄的增长她的信心增强了。（引导时间状语从句）

(b) She may need some help as she's new. 她是新来的，可能需要一些帮助。（引导原因状语从句）

(c) Try as he might (= however hard he tried) , he couldn't open the door. 他想尽了办法也没能打开门。（引导让步状语从句）

(d) They did as I had asked. 他们是按照我的要求做的。(引导方式状语从句，as=in the way in which)

第二种是用在“as+(形容词或副词)+as”中，其中第一个as为副词（见下文），第二个as为连词，引出关联对象：

You're as tall as your father. 你和你父亲一样高。

I haven't known him as long as you (= as you have known him).我认识他的时间没有你长。

三、as用作副词：

第一种是用在“as+(形容词或副词)+as”中，其中第一个as为副词（例句见上， as分别修饰形容词tall和副词long）

第二种表示以特定形式或关系考虑——通常用在介词或分词前

The ‘h’ in honest is silent, as in ‘hour’. honest中的h与hour中的h一样都不发音。

my opinion as distinguished from his：我的意见与他的不同

As always, he said little. 他和平时一样，少言寡语。

第三种表示举例：various trees, as oak or pine（同such as）

四、as用作代词（关系代词）：

(a) 关系代词as引导限定性定语从句：一般用于以下固定结构中：

1. 用于the same…as结构中，例如：

This the same book as I read last week.

2. 用于such…as结构中

I don’t like such books as he recommends.

3. 用于“so+ adj +a/an +n + as”结构中

I am not so strong a man as I was.

即：当先行词含same、 such、so等时，关系代词要用as。

(b) 关系代词as引导非限定性定语从句：先行词是一个句子或短语:

As is often the case, she is late. (放在句首，先行词是整个句子she is late)

He is very careful, as his work shows. (放在句末，先行词是整个句子he is very careful)

To shut your eyes to facts, as many of you do, is foolish. (放在句中，先行词是不定式短语To shutyour eyes to facts)

类似的表达还有：As we all know,as is known to all, as is mentioned/discussed above, as we have mentioned/discussed above

它与which引导非限定性定语从句修饰前面整个句子的区别在于：which只能放在句末，而as可以放在句首、句中和句末。