What fun it is to play .这里的what为什么不能用how?

What fun it is to play .这里的what为什么不能用how? what fun it is to swim 为什么不能用how? what fun it is to listen music 这里的fun为什么不能是形容词用how fun. __ it is to play with snow in winter!A How fun B what a fun 为什么是what fun it is to. 为什么是what fun it is to.而不是how funn it is to. 为什么能说What fun it is to do sth.却不能说How fun it is to do sth. what ___(funny) it is to play with children!横线处填fun为什么? （）fun it is to have ice cream in hot summer!填what还是how 为什么? how fun it is to do … what fun it is to do… 为什么要用后者,fun如果当形容 what fun it is和how fun it is 为什么说what fun it is!而不用how fun it is?这是什么句型?