However,ashiscareerwasbeginning,CaisparentssenthimtotheUnitedStatestostudymusic.

I saw him as he was getting off the bus.

begin和set off是短暫性動(dòng)詞吧，為何不是一般過(guò)去時(shí)？?jī)删涠加昧薭e doing，句子該如何理解呢？as后面接短暫性動(dòng)詞和延續(xù)性動(dòng)詞都可以嗎？?jī)删湓?huà)中的as可以換成when或while嗎?謝謝！

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、延續(xù)性動(dòng)詞的進(jìn)行體，表示動(dòng)作正在發(fā)生的過(guò)程中，而終止性動(dòng)詞的進(jìn)行體，從嚴(yán)格意義來(lái)講，表示動(dòng)作尚未發(fā)生，意味著“馬上發(fā)生；就要發(fā)生；剛要發(fā)生”。大家在理解時(shí)，務(wù)必注意這個(gè)問(wèn)題。劃線(xiàn)部分，只須句意允許可以用一般體（一般過(guò)去時(shí)），表明動(dòng)作已發(fā)生。

作一對(duì)比：

I saw himas he was getting off the bus.（還沒(méi)有下車(chē)）

本句意味著：在他就要下車(chē)/剛要下車(chē)之時(shí)，我看見(jiàn)了他。

I saw himas he got off the bus.（已下車(chē)）

本句意味著：他剛剛下了車(chē)，我就看見(jiàn)了他。

以上兩句，可以用 when 代替 as。

至此，已經(jīng)回答了網(wǎng)友的第一個(gè)問(wèn)題：為何不是一般過(guò)去時(shí)？

2、關(guān)于第二個(gè)問(wèn)題：兩句話(huà)中的 as 可以換成 when 或 while 嗎?

在這兩句中，可以用 when 替換 as ，但 while 不能。由于 while 引導(dǎo)的時(shí)間從句，謂語(yǔ)動(dòng)詞需要是延續(xù)的。而 as 和 when 可以延續(xù)，可以不延續(xù)。當(dāng)主、從句都用了不延續(xù)的動(dòng)作動(dòng)作時(shí)，as 和 when 都表示兩個(gè)動(dòng)作幾乎是同時(shí)發(fā)生。但，相對(duì)而言，as 表示的主從句的兩個(gè)動(dòng)作愈加緊湊！when 略有遜色。

正誤判斷如下：

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.（誤）

I saw him while he was getting off the bus.（誤）