as作連詞的用法:

1. 作"當……的時候",引導時間狀語從句。注意與when、 while的用法區別。
① when作"當……的時候"解,可以指較短的(一點)時間,也可指一段時間。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常表示一段較長的時間或一個過程,強調主句謂語動詞與從句謂語動詞同時發生。如:
Work while you work. Play while you play.
③ 但屬下列情形時,只用as, 而不用when或while。
A) 用于表示同一個人的兩種動作交替進行,指"一邊……,一邊……"。如:
The girl dances as she sings on the stage. 
He looked behind from time to time as he went forward.
B)表示兩個同步發展的動作或行為,譯為"隨著……"。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)表示兩個短促行為或事件幾乎同時發生。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) 后接名詞表示某一年齡段時,用as。如:
As a young man, he was active in sports. 

2. as=since, 作"既然"、"由于"解,引導原因狀語從句,常用來表示已為人們所知或顯而易見的原因或理由。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,引導方式狀語從句。如:
He speaks English as Americans do.

4. 用于as...as...或not so/ as...as...中,前一個as是副詞,后一個as是連詞,引導比較狀語從句。如:
They helped the old man as often as possible. 
I don''''t speak English so/ as well as he does. 

5. 作"雖然"、"盡管"解,引導讓步狀語從句。常用倒裝語序,即adj. /adv. / n.+as +主語+謂語+主句。如:
Tired as they were, they walked on.
Child as he is, he knows a lot. (child前不用冠詞)

They stopped to buy parchment and quills.

stop 用法

stop doing sth是“停止做某事”，doing是stop 的賓語，是要停止的動作，
stop to do sth 是“停下來開始做某事”，to do是stop的狀語，表示目的，是要開始做的事情，例如：

1. The two girls stopped talking when they saw me.
那兩個女孩一見到我就停止了講話。

2. The two girls stopped to talk to me when they saw me.
這兩個女孩一看到我就停下來和我講我。