请分析这句英语的语法The cave might never have been discovered had not

请分析这句英语的语法
The cave might never have been discovered had not the entrance been spotted by the distinguished French potholer,Berger
请分析这句话的语法,中文意思我知道,关键是什么语法结构.

此句是省略了 if 的虚拟语气句型.
虚拟语气中,if 条件状语从句如果把 if 省略,就要把从句里的 were,should,had 这三个词放到主语前,形成倒装.如果 if 从句里没有这三个词当中的任意一个,则不能倒装.
主句是：The cave might never have been discovered
从句是：had not the entrance been spotted by the distinguished French potholer,Berger
恢复 if,则从句为 If the entrance had not been spotted by .
原句中的 not 按照我所学的语法是不要放到主语前面去的.
更多知识点建议参考语法中的虚拟语气专题和倒装专题.