after a while 不久,过一会儿 After a while,he was asleep.
in a while adv.过一会,过不久 once in a while 偶尔
She haves a headache once in a while
for a short while 暂时 短短一会儿 The road is closed to traffic for a short while.
区别：after a while 可以看做是个时间状语是某一时间点 after + 一段时间 = 一段时间 + later
in a while 通常是 once in a while 的形式 表示not often
for a short while 相当于时间段,表示持续的过程