The present subjunctive is used when referring to the present or the future. If we're referring to the past or conditional scenarios, we use the IMPERFECT SUBJUNCTIVE.
1. Make the 3rd person plural form of the preterit: ESTUDIARON
2. Take off the -ON: ESTUDIAR-
3. Add the ending: que tú ESTUDIARAS
Again, any irregularities that happen in that initial preterit form will carry through to the imperfect subjunctive.
Below is a video I made showing the formation of imperfect subjunctive.
Acceso also has a good page on Imperfect Subjunctive with another video. You may notice it has very similar explanations for the uses of the imperfect subjunctive. That's because the functions of subjunctive are the same no matter what tense you're dealing with.