playing就是动名词,动名词通常放句首做主语,例如这句话可以变成:
Playing football is always my favorite.(踢足球总是我的最喜爱的)
类似的例子还有:
Teaching English is her lifelong career(教英语是她终身职业)
Seeing is believing(眼见为实)
play football的动名词
play football这个词的意思是玩足球,写改为动名词形式,只需要在动词之前家长ing即playing football。