He met many problems__he was going over his lessons 为何用while
问题描述：
He met many problems__he was going over his lessons 为何用while
while不是前后都要进行时么
答
He met many problems__he was going over his lessons
While 是后边必须是进行时,前边可以是,可以不是.如果前边的动作时短暂的就不用进行时.