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.
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