keep on doing. 这里的doing是做on的介词宾语还是keep on的后接宾语，keep on为短语动词？

keep on doing
持续/继续做（doing 是介词on的宾语）
keep doing
继续做（doing 是keep的宾语）
二者可互换。 keep on 是强调决心及动作反复持续的时间，可已有时间间隔，但 keep doing是指连续不断的做事情，不能有时间间隔。

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.

ith four wheels, which was put in motion with a handle wroug