下面两个都可以，关键看你想表达什么内容。
keep doing sth, 如：He keeps smiling.
keep on doing sth, 如：He keep on ring his grilfriend up.
keep后面加..,表示"坚持做某事
下面两个都可以，关键看你想表达什么内容。
keep doing sth, 如：He keeps smiling.
keep on doing sth, 如：He keep on ring his grilfriend up.