Several times in the current chapter of Fabulae Syrae is the future perfect used as an imperative (“Ne hoc feceris”). Gildersleeve mentions that this usage exists but not more. Is there any real distinction between using the regular imperative and the future perfect? He also mentions the future as imperative, which is idiomatic in English (“You will not do this” or, better, “shall”); I guess the future perfect would be “do not be in the state of having done this”?
I agree with truks. Better to think of this as perfect subjunctive, unless you feel there really is a good reason to think otherwise. That’s how it’s generally analysed.
It is true that Latin has many ways of expressing commands and prohibitions, often with slightly differring connotations, like cura, ut ualeas.
The only knotty thing is that as I learned it – and in every paradigm I’ve checked – a short I, as in my example, indicates a future perfect while a long one indicates a perfect subjunctive. (I probably should have used macrons, as my book does). I had thought perfect subjunctive at first but it seemed odd for a past tense to form a present or future command; in every instance in the Fabulae Syrae the I in the future perfect/perfect subjunctive is short. Yet A&G uses a short I to form the perfect subjunctive in their examples. I’m not sure what to think.
But the long ī’s of coni. perf. started to shorten already in Archaic Latin (-īt -īnt > -ĭt -ĭnt occurred first), and in Classical Latin the parallel personal forms in these paradigms had formally merged except for the 1st sing. This means of course that ī would prove coniunctiuus perfecti, but ĭ doesn’t prove futurum exactum.
(In fut. ex. one might expect fēcĕrunt in the 3rd pl., but there has probably happened levelling from some direction.)