as可以作连词、介词及关系代词。在英语四级考试中经常会用到，现在，小编现将其用法小结如下，大家可以作为参考。

一、 as作连词的用法:

1. 作"当……的时候",引导时间状语从句。注意与when、 while的用法区别。

① when作"当……的时候"解,可以指较短的(一点)时间,也可指一段时间。如:

When he was at college, he could speak several foreign languages.

When the clock struck twelve, all the lights in the street went out.

② while常表示一段较长的时间或一个过程,强调主句谓语动词与从句谓语动词同时发生。如:

Work while you work. Play while you play.

③ 但属下列情形时,只用as, 而不用when或while。

A) 用于表示同一个人的两种动作交替进行,指"一边……,一边……"。如:

The girl dances as she sings on the stage.

He looked behind from time to time as he went forward.

B)表示两个同步发展的动作或行为,译为"随着……"。如:

As time went on / by, she became more and more worried.

As children get older, they become more and more interested in the things around them.

C)表示两个短促行为或事件几乎同时发生。如:

I thought of it just as you opened your mouth.

Just as the flying worm hit her face, she gave a loud cry.

D) 后接名词表示某一年龄段时,用as。如:

As a young man, he was active in sports.

2. as=since, 作"既然"、"由于"解,引导原因状语从句,常用来表示已为人们所知或显而易见的原因或理由。如:

As / Since you''''re not feeling well, you may stay at home.

As he wasn''''t ready in time, we went without him.

3. as=in the way that, 作"像"、"按照……的方式"解,引导方式状语从句。如:

He speaks English as Americans do.

Remember, you must do everything as I do.

4. 用于as...as...或not so/ as...as...中,前一个as是副词,后一个as是连词,引导比较状语从句。如:

They helped the old man as often as possible.

I don''''t speak English so/ as well as he does.

5. 作"虽然"、"尽管"解,引导让步状语从句。常用倒装语序,即adj. /adv. / n.+as +主语+谓语+主句。如:

Tired as they were, they walked on.

Child as he is, he knows a lot. (child前不用冠词)

以上就是内容就是关于英语四级语法知识，希望可以给大家在备考的时候带来帮助。