Wow - I have not seen "tuvieren" "hicieren" etc. in about 500 to 600 years.
Yes, it exists in antique literature and some legal documents, as the link mentions, but never in real life nowadays. Future subjunctive is just telescoped into present subjuntive nowadays for anything that hasn't happened yet.
Right now, Faltan 2 minutos para que sean las 8:00 -- who knows, maybe the world will end in the next two minutes, and we'll never get to 8:00!!!!
But they're not equivalent; consider these two examples:
Example 1: "What do you think we'll eat tonight?" A good answer would be "Perhaps we will eat crab." To answer with "Perhaps we could eat crab" sounds rather strange.
Example 2: "Shall we eat steak tonight?" Now "Perhaps we could eat crab" is a response, but "Perhaps we will eat crab" is a highly unnatural answer.
The key difference is that when you add "could" you imply that you think the other person has control of the result. The first example sounds strange because the person asking "what do you think we'll eat tonight" does not appear to be in control (unless he/she is playing games with you). The second is strange for the opposite reason.
So Duolingo is correct not to accept "Perhaps we could eat crab." It does not mean the same thing as "Perhaps we will eat crab" nor is it a reasonable translation for "Quizás comamos cangrejo."
I recommend reading "A New Reference Grammar of Modern Spanish: Fifth Edition" (Butt and Benjamin, 2011, section 26.3.2 "The Subjunctive after words meaning 'perhaps', 'possibly', 'probably' )
For me "Perhaps we eat crab" tries unsuccessfully to straddle the line between awkward and just plain wrong. It's the kind of thing that sounds charming from a second-language speaker, but would never be said by a native speaker. It's either "Perhaps we are eating crab" or "Perhaps we will eat crab."
@perlafantastica; as chilenaDcorazon2 mentioned already "comamos" is a form used when you are not sure of something (grammatically speaking it is the "subjunctive"); the problem is that the English language has only very few subjunctive forms (as in the sentence: if I WERE you....(expressing uncertainty) instead of "if I was you" (expressing certainty), so you use a work-around, which of course is not really a one-on-one translation.
I understand that you do not like "Maybe we will eat crab" which Duolingo considers correct. The translation I got was "we might eat crab", which to my mind should be translated as "Podríamos comer cangrejo". I suggested "perhaps we may eat crab", but Duolingo did not like it either :-(, never mind; I hope my explanations help :-)
Shouldn't this be subjunctive future? http://www.spanishdict.com/answers/100059/future-subjunctive#.UWxqjLVg9ys
I am royally confused about the subjunctive when referring to tenses other than the present. Can someone help to explain how one would say the following?
Maybe we are eating crab? Maybe we ate crab?
Also, the present and future tense of the subjunctive is the same? So we can only tell which one is being used by the context?
That's not how you use "maybe" in Spanish, with a subjunctive; it's a double "perhaps".
In Spanish, you can either have one or the other, but not both. "Quizás comemos" or "Comamos" are both correct, but "Quizá comemos" is literally "Maybe we'll probably eat", and that's not something people say in Spanish.