虚拟语气TODO
Q: 对现在虚拟和对未来虚拟，从语法上讲好像差不多。
对过去虚拟，从句had,主句have done。
不论语言和语法，要表达的意思就是一个意思
if条件状语从句
过去事实的虚拟
从句过去完成时(had+done)，主句(should,would,could,might) + (have+done) 现在完成。
对过去的事实进行虚拟 + 从句助动词倒装
- The accident wouldn’t have happened had the weather been better.
- Had he put on his seatbelt, he might not have been injured so badly.
