有二种情况:
一、动词作名词用。
二、动词用在进行式时态中。
1.在
现在进行时后面加ing;如:I am watching TV.
2.
be动词后面动词加ing;如:They are doing homework.
He is doing homework.
3.like;love;enjoy(享受)后面加ing;如:He likes piaying football.
4.当动词在短语中作主语时动词后加ing; 如:Playing football is very interesting(有趣).
5.介词后面也要加ing;如:about reading books.
6.
过去进行时也要加ing.如:I was sleeping when it started to rain.
7.一些特定词组要加ing.如:be afraid of doing;do some cooking /cleaning/reading等.
8.
一般将来时用现在进行时代替将来时用ing,如:I am going to out.当将来时用will时,不加ing.