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）
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