swim是动词,只能做谓语
swimming可看成动名词,即名词,可作主语、be swimming作谓语,swimming还能做伴随状语
例如 he's thinking of something,swimming in the pool.
swimming还能做后置定语,修饰前面的名词
如bread is eaten by a fish swimming in the pool.
基本上所有的都可以这样用
选我哦 我说的绝对没错