keep on doing 和keep doing 的区别

keep doing / keep on doing
1) 均可表示“继续做某事，反复做某事”， 可互换，只是 keep on 更加强调时间的间隔性和动作的反复性，及某人做某事的决心。
He kept on working until midnight though he was tied.
The boy kept /kept on asking some silly questions in class.
2) 若表示一连续不段的动作，或某一动作的持续状态，只宜用keep doing .
She kept waving to her husband until he was out of sight.
They kept standing outside the room during all the meeting.
Keep lying here; I will be back soon.
3) 若表示今后或将来继续做某事，则只宜用keep on doing 。
We must keep on working hard in the coming new year.
He kept on smoking though we advised him to stop to smoke.

来源http://web.etiantian.com/staticpages/study/question/question_1486524.htm

两者都是“持续做某事”的意思，但是有细微的差别：
keep on doing是指在做着一件事情，中间有段空隙时间（做别的什么事了），然后继续做
keep doing 是指，一直在做某事，没有停止过

be kept dry
保持干燥

The old lady kept on about her illness.
那老太太没完没了地讲她的病情。