for a time 有一段时间,一度
如for a time ,he had everyone`s attention .他一度得到所有人的注意.
on a time 从前,副词
Think back on a time when you did this .回想以前你这么做的时候.
at a time 每次
He tossed the peanuts into his mouth one at a time.
他每次将一粒花生扔进嘴里.