What 修饰的是形容词性的名词.

e.g.What an interesting story he told us!

句子结构：What+(a/an)+形容词+名词+主语+谓语.

how修饰的是形容词或副词.

e.g.How well she sings!How happy they look!

句子结构：How+形容词/副词++主语+谓语.

What修饰的是可数名词复数.

What a 修饰的是可数名词单数或不可数名词.

What an 修饰的是元音开头的可数名词单数或不可数名词.

How 一般后面不加a或an.