The subjunctive mood refers to something that didn't happen, and you fictitious it. Usually divided into two situations.
1. Fiction about what happened in the past means that you are not satisfied with what happened in the past, and now you make fiction about it.
If I study English hard, I will pass the postgraduate exam.
The usual format is: the clause is in the form of had done, and the main sentence is in the form of would/could/should had.
2. The virtuality of things that didn't happen means that you have good wishes for things that didn't happen.
The usual format is that the clause is in the form of could/would/should do, and the main sentence is in the form of could/could/should do.
In short, I hope it will help you.