he was playing in the street 中的 in the street是介词短语,可以省略；省略后变成he was playing ,这是过去进行时,表示过去正在发生的事,而he is playing 是现在进行时,表示现在正在发生的事. 至于sb was doing sth 是否成立,你体会一下下面的句子： he was playing in the street.他在街上玩. he was playing basketball in the street.他在街上玩篮球.
