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.

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)

They stopped to buy parchment and quills.

stop 用法

stop doing sth是“停止做某事”，doing是stop 的宾语，是要停止的动作，
stop to do sth 是“停下来开始做某事”，to do是stop的状语，表示目的，是要开始做的事情，例如：

1. The two girls stopped talking when they saw me.
那两个女孩一见到我就停止了讲话。

2. The two girls stopped to talk to me when they saw me.
这两个女孩一看到我就停下来和我讲我。