what a clever boy he is为什么不用how a那什么时候该用what a 什么时候该用how a请说明举例子

因为这是一句感叹句，感叹句以what a （an)开头。
What is the weather like today?
How 's it going?

修饰名词时用what,修饰形容词时用how. what a修饰可数名词，不可数名词直接用what修饰，没有how a的用法。例如：
What a beautiful girl she is!
What exciting news it is!
How delicious it tastes!

修饰名词时用what,修饰形容词时用how,句中用the.
what a beautiful flower.how beautiful the flower is.

clever boy 是名词用what，how clever the boy is！