He plays the piano as well as Mary does,前一个as是副词,后一个as是连词,引导比较状语从句,所以比较的东西应该是一样的.不能够拿he plays the piano跟Mary作比较.直接加人名的情况,比如说：She is as beautiful as her sister.
as可以作连词、介词及关系代词.现将其用法小结如下:
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)
二、 as作关系代词,引导定语从句,作"正如"、"这一点"解.如:
He is very careful, as his work shows.
As is well-known, Taiwan belongs to China.
as的这种用法,请看2001年的高考题:
is known to everybody, the moon travels round the earth once every month.
A. It B. As C. That D. What
字母下划横线的为正确答案)
另外,当先行词被the same, such等词修饰时,常用as来引导定语从句.如:
Such a clever boy as he can learn anything quickly.
I was reading the same book as he bought yesterday.
三、 as作介词的用法:
as引导的介词短语大多用作状语,as译为"作为";少数情况可引起宾语补足语.如:
As a League member, I'll take the lead in everything. (状语)
She works as a doctor. (状语)
They have me as one of their own children.(引起宾语补足语)