1.当一个句子已经有一个动词了
2.一些固定的词和词组后面比如
feel like doing risk doing it is no use doing
列举一些常见的后面加doing的:keep enjoy excuse practice mind avoid
appreciate risk consider can't help give up imagine finish think of suggest hear of be fond of insist on aim at 其实就是大多数介词后面~(我可是一个一个打上去的...)
3.表示主动
4.表示进行
我不知道全不全 至少这些很常用