1动名词。例：Swimming is good for health.
2现在进行时。例：I am doing my homework.
3动词并列。例：She stood here, reading a book.
4动词前有部分介词。例：She is looking forward to seeing him.
5有些动词加ing变为形容词。例 ：exhaust（动词：耗尽精力）加ing为exhausting（形容词：精疲力尽的）
当然还有很多情况：
1、表将来的时候,例如：spring is coming 就要到春天了.2、某些介词后面需要跟动词的ing形式,例如：I‘m interested in studying japanese3、作表语或动名词时要用ing
例如：Her job is washing,cleaning and taking care of the children.他的工作就是清洗、整理并照顾好孩子 living is to die 离开意味着死亡4、作定语一般表示所修饰名词事物的用途的时候.例如：
a writing desk=a desk for writing 写字台
a swimming pool=a pool swimming 游泳池
