As 的用法（一） as 作连词的用法: 1.作“在。。。期间，当。。。的时候”引导时间状语从句. as 与 when，while 都是引导时间状语从句的从属连词，含义都是"当……的时候"。但它们有区别：用 when 时，从句的动作可以与主句的动作同时发生，也可以先于主句的动作发生；用 while 时，从句的动作为一过程，主句动作与从句动作同时进行或在从句动作过程中发生；用 as 时，主句和从句的动作同时发生，具有延续的含义。 ① 下列情形时,只用 as, 而不用 when 或 while。 1)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 sings as she goes to school. He looked behind occasionally as he went forward in the forest. 2)表示两个同步发生的动作或行为,意思是随着-----的发展.如: As time went on / by, she became more and more beautiful . As children get older, they become more and more interested in everything. 3)表示两个短暂行为或事情几乎同时发生.如:I watched her as she read the book. I thought of it just as you opened your mouth. Just as the flying worm hit her face, she gave a loud cry. 4)接在名词后面表示某一个年龄段时.如: As a young man, he was active in sports. ②when 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 I was at college, I could speak several foreign languages. When the clock struck twelve, I had slept for 2hours. ③ 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. 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 he’s been ill for ages, I will help him. As / Since you're not feeling well, you may stay at home. As he wasn't ready , we went without him. as，because，since 都可以表示因果关系，连接原因状语从句，含义是因为，由于，但它们有区别：because 表示的语气最强；as 一般放在句首，语气较弱，较口语化；since常常用在书面语中，表示多为对方已知的、或稍加分析便可得知的原因，有...
