感叹句可以用how也可以用what
what a/an + 形容词+可数名词单数 +主谓!
what +形容词+可数名词复数/不可数名词 +主谓!
How +形容词+主谓!
e.g.What a beautiful flower it is !
What beautiful flowers they are!
How beautiful the flowers are!
fun可以做不可数名词,表示有趣的人或物.所以用what而不用how