It's a very difficult subject. I, for one, do not believe we have a future tense, but many disagree and claim 'will' is the future tense.
The way to prove it is through demonstrating that it is not inflected like the opast tense usually is, there are many other ways of talking about the future and 'will' doesn't always serve to indicate the future. However, having been through and seen this argument time and time again, I think of it as more a question of faith\belief, rather than logic.
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