由if引导的虚拟语气的三种用法？

Answer 1

有三种情况：
（1）对过去,从句had done主句would/could/might/have done
（2）对现在,从句：did /were主句：would/could/might do
（3）对将来,should do/were to do/did.从句,更上一个一样.注意,“主将从现”原则.