his success is entirely due to hard work为什么entire用副词形式而不是形容词 它修饰什么?

因为是做状语修饰介词短语due to hard work的
due to hard work 这个介词短语在句中作表语
 
注意【副词+ 介词短语】结构：
He is well above fifty.他远远超过50岁.
It is far below the ordinary level.这个远远低于一般的水准.
He worked deep into the night.他工作到深夜.