|与过去事实相反||had done||should/would/might/could+have +done|
|与现在事实相反||did (be用were)||should/would/might/could +v原形|
|与将来事实相反||did (be用were) / should do
were to do
If I had super power, I would turn time back.
If you had left your bicycle unlocked last night, someone might have stolen it.
(1)表“命令”(order, command, instruct)、“建议、提议”(suggest, advise, propose, advocate, recommend)、“请求、要求”(request, require, urge, demand, insist)等词汇后跟从句时，从句须用虚拟语气。结构为：(should) + v.原形。如：
(should) be transferred into market economy. (动词+宾语从句)
His demand is that we (should) set off at once. (名词+表语从句)
(2)It be + 形容词 + that... (should)...。用于该句型的形容词有：vital, necessary, imperative, important, essential, better, natural, proper, surprising等。如：
It was imperative that he (should) act as naturally as possible.