as 是副词,介词,和连词；在一些语境下 as 可以接 动词+ ing；比如当 动词+ing = 形容词

连词conj. 因为；随着；虽然；依照；当…时，如：

As Napoleon once said, attack is the best method of defence.

拿破仑曾经说过，进攻是最好的防御。

介词prep. 如同；当作；以…的身份，如：

As a young man, Eliot had studied art in Paris.

年轻时，艾略特在巴黎学画画。

副词adv. 同样地；和…一样的，如：

Tom's not as old as you.

汤姆年纪没有你大。