how a ,how ,what a ,what 之间怎么使用

一般原则what后加名词短语（不论是可数名词还是不可数名词,也不管是可数名词单数还是复数）
how后一般跟形容词和副词.
例如：
what good weather It is today!
What good children they are!
What a nice gift!
How fast he is running!
How beautiful these flowers are!
但是当修饰一个单数可数名词且同时名词前有形容词修饰时,what和how可以转换(注意必须满足条件才可以转换!）例如：
What a difficult problem it is!= How difficult a problem it is!