1.进行时 be+ v-ing
2.动词做主语时 如：Watching TV too much is bad for your eyes.
3.位于介词后面时 如：for/in/with/at...+v-ing
4.短语固定搭配 如：stop doing sth.
5.特定动词后 如：finish/mind/enjoy/spend/avoid/...doing sth.