Ving如果作主语,那么应该是动名词,这是个动词,但是在句子中要起名词的作用,因此,为动名词.而作谓语时,ving为现在分词,或也可是动名词,区别在于：
He is swimming.
(He 为主语；is swimming为谓语)
His favorite sport is swimming.
(His favorite sport 为主语；is 为系动词；swimming为表语；is swimming为谓语)系表结构一起构成谓语部分.
补语：
ving作定语修饰名词时是有区别的：现在分词的形式和动名词相同,都是在动词后面加ing,但区别在于：
a sleeping child (=a child who is sleeping)正在睡觉的孩子
a sleeping car (=a car for sleeping) 卧铺车
也就是说,现在分词作定语修饰名词,表示名词的动作；而动名词作定语修饰名词则强调该名词的作用.
a swimming girl = a girl who is swimming (游泳的女孩)
a swimming pool = a pool for swimming (游泳池)