forget to do sth。和forget doing sth. (remember to do sth. 和remember doing sth。)
forget to do sth。表示将来不要忘记做某事，谈的是未来的事情;forget doing sth。表示忘记过去应该做的事情。
例如： Dont forget to do your homework。 said the teacher before the class was over.
老师在下课前说：不要忘记做家庭作业。
Im sorry. I forgot doing my homework. May I hand it in this afternoon, Mr. Chen? said Li Ming.
李明说：对不起，我忘记做家庭作业了。我今天下午交好吗，陈老师?