future and subjuntivo

Question

Now I am confused about the use of Presente de subjuntivo versus futuro. If the condition is presently probable or future probable, then the future is used? If the present condition is unlikely or not probable, then I can use the subjuntivo? So the issue is the certainty or uncertainty of the condition? John will win/John (is certain to win): Juan ganará. I hope that John will win: Espero  que John gane.

Inma Sánchez · Accepted Answer

Hola John
I think your confusion comes from part of the lesson, where it may sound as if the future tense and the present subjunctive are interchangeable, but they aren't. (I will re-word that part of the lesson to make it clear)
What we mean is:
The future tense can itself express the speaker’s guess, prediction or probability. The subjunctive does not, by itself, express this. It appears because another expression of probability, doubt, possibility, etc. requires the subjunctive, e.g. "es probable que...", "dudo que..."...
So compare:
1. "Estarán ocupados." = They’re probably busy / I suppose they’re busy. (the speaker's mindset is "I am just guessing, imagining, expecting... they are probably busy because they haven't arrived yet)
2. "Es posible que estén ocupados." = It’s possible that they’re busy. The possibility here is explicit with "es posible que". 
 
In the first sentence, estarán is doing the speculative work. In the second, es posible que expresses the possibility, and estén is subjunctive because the proposition is presented through es posible que.
Does this clarify your doubt? 
If you need us to expand on this please let us know. 
Saludos
Inma