Future is easier for a student to master. For all verbs it's the same as the subjunctive form of the verb + गा/गे/गी. No exceptions, as this formation is regular.
However, some subjunctive forms are irregular for some common verbs. But luckily they are just 4, If I remember correctly. होना (to be/happen), लेना (to take), देना (to give) and पीना (to drink).
होना is the only verb with unique present and future subjunctive forms and for all other verbs there is just one form which is used as both present and the future subjunctive. The future subjunctive is regular but the present subjunctive is irregularly formed.
Past also have very few irregular forms. Only 5 if I remember correctly. Add the verb करना to the list I gave above and that's it. Only these are irregular. Past is more complicated because there are different types of past in Hindi. Perfective past, Habitual Past, Simple Past and Progressive Past.