THE FUTURE SUBJUNCTIVE?
Latin doesn’t have a future subjunctive tense, but it does need a way to specify future action in a variety of subordinate clauses. In the indirect question ‘I asked what you what you were going to do’, ‘going to do’ needs to be in the future, that is, the action needs to be marked as following, subsequent to, the time of the main verb.
To solve this impasse Latin improvises by inventing the second of its two periphrastic constructions, the so-called future active periphrastic. As with first of these constructions – the passive periphrastic (a gerundive + esse, denoting obligation) – the name is more complicated and confusing than the thing itself.
Simply put, if you want to specify future action in a subordinate clause, take the future active participle (the -urus ones) and add the relevant subjunctive form of esse: present subjunctive for primary sequence, imperfect subjunctive for historic sequence.
It will look something like this:
rogo quid facturus sis – I’m asking what you’re going to do
rogavi quid facturus esses – I asked you what you were going to do.
This future active periphrastic construction is definitely towards of the more difficult end of the Latin grammar scale: you will meet it only in the specific context of future actions in subordinate clauses. Don’t consider this a priority unless you’re already comfortable with the subjunctive, with subordinate clauses, and with the sequence of tenses. As usual, it’s best learned by meeting it in context, and you can check in here as often as you wish.