Doing exercise is good for your health .和 Swimming is good exercise.这两句话为什么要在动词加ing?

问题描述:

Doing exercise is good for your health .和 Swimming is good exercise.
这两句话为什么要在动词加ing?

这里的动词+ing 成为动名词,在语法中作名词用。

因为是动词做主语 放在句首 所以要用动名词的形式