as
prep.（介词）
(表示时间)当还是…的时候
As a schoolboy,he showed every sign of genius.
当他还是个小学生的时候,就显示出了天资聪慧.
(表示方式)以…的身份,以…资格,作为; 以…形式,以作为; 以…角色,扮演…角色; 如同,像
(表示结果)成为,看作,看成
We had better treat it as a joke.
我们最好把它当作玩笑.
(表示目的)为了,以…为目的
The dykes were built as a protection against the sea.
建筑堤坝是为了防止海水泛滥.
(表示举例)例如,诸如…之类的
We talked about such subjects as the weather.
我们谈论了诸如天气之类的话题.
conj.（连词）
在…期间,当…时候
I was coming in as he was going out.
我进来的时候,他正出去.
尽管,即使,虽然
Intelligent as you are,I suspect you will fail.
尽管你聪明,我猜想你会失败.
像…一样
由于,因为
As you weren't there I left a message.
因为你不在那里,我留了个信儿.