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.
