he was knocked down(when crossing the street).

题目

he was knocked down(when crossing the street).
请问括号中为什么可以这样用?

答案

这就是语法的神奇.这句原来是这样的 when he crossed the street 省略主语he后,动词须变为ing形式.当他过街的时候,他被绊倒了.