However,ashiscareerwasbeginning,CaisparentssenthimtotheUnitedStatestostudymusic.
I saw him as he was getting off the bus.
begin和set off是短暂性动词吧，为何不是一般过去时？两句都用了be doing，句子该如何理解呢？as后面接短暂性动词和延续性动词都可以吗？两句话中的as可以换成when或while吗?谢谢！
However,as his career was beginning, Cais parents sent him to the United States to study music.
I saw himas he was getting off the bus.
1、延续性动词的进行体，表示动作正在发生的过程中，而终止性动词的进行体，从严格意义来讲，表示动作尚未发生，意味着“马上发生；就要发生；刚要发生”。大家在理解时，务必注意这个问题。划线部分，只须句意允许可以用一般体（一般过去时），表明动作已发生。
作一对比：
I saw himas he was getting off the bus.（还没有下车）
本句意味着：在他就要下车/刚要下车之时，我看见了他。
I saw himas he got off the bus.（已下车）
本句意味着：他刚刚下了车，我就看见了他。
以上两句，可以用 when 代替 as。
至此，已经回答了网友的第一个问题：为何不是一般过去时？
2、关于第二个问题：两句话中的 as 可以换成 when 或 while 吗?
在这两句中，可以用 when 替换 as ，但 while 不能。由于 while 引导的时间从句，谓语动词需要是延续的。而 as 和 when 可以延续，可以不延续。当主、从句都用了不延续的动作动作时，as 和 when 都表示两个动作几乎是同时发生。但，相对而言，as 表示的主从句的两个动作愈加紧凑！when 略有逊色。
正误判断如下：
However,as/when his career was beginning, Cais parents sent him to the United States to study music. （正）
I saw himas/when he was getting off the bus.（正）
However, while his career was beginning, Cais parents sent him to the United States to study music.（误）
I saw him while he was getting off the bus.（误）