Question

should do 和后退一个时态都是虚拟语气的使用形式吗?那么何时用should,何时后退时态?

Answer 1

有三个时态的虚拟语气,都有用到should
（if引导的）从句中的v.主句中的v.
现在 did / were should / would / might / could + do
过去 had done / been should / would / might / could + have done / been
将来 were to / should + do should / would / might / could + do
给你造几个句子,看看能不能更清楚：
现在 If I were a train,I should travel to all the splendid scenes in this planet.
过去 If you had not been a queer,you should never have broken your mom's heart.
将来 If our team were to undermine the opponent's reputation for our own good,we should be sorry.