保持做某事 只有keep on doing sth.有没有keep doing sth.或keep to do sth.

keep on doing sth,继续做某事,有决心、坚持等主观意念在内.
keep doing sth,继续做某件事或某个动作,没有主观意念在内.
keep to do,没有这种用法,只有keep to sth,遵守或坚持某件事,如keep to one's promise.