问一个英文句式

题目

问一个英文句式
The cave might never have been discovered had not the entrance been spotted by the distinguished Frech pot-holer,Berger.
其中的 had not 是用来作神马的?这是什么句式呢,看不懂

答案

might never have been diacovered 是前面The cave 的定语.意思是这可能还没有被发现的洞穴
The cave(that/which) might never have been discovered 中省了THAT 或which.
The cave might never have been discovered 是主语 had not 谓语 entrance been spotted by the distinguished Frech pot-holer,Berger.宾语（ been spotted by the distinguished Frech pot-holer,Berger是entrance 定语）