1、正在进行时中,谓语动词后加ing.如:

I'm playing football.

2、当动词做主语时,要用动名词,即动词加ing.如:

Making salad.

3、一些特殊表达.动词后再用动词要用动名词.如:

enjoy doing sth. practice doing sth. mind doing sth. finish doing sth. keep doing sth.

享受做某事，练习做某事，介意做某事，完成某事，继续做某事。