This will blow Arga’s mind, but there is no future tense in English, or in any Germanic language for that matter.
What English has is a variety of ways to express futurity. The most common, the one that is commonly called “future tense,” is the periphrastic construction using will or shall. We can also express the future adverbially, as in: “Tomorrow I have a doctor’s appointment.” What we don’t have are inflections of verbs that signal futurity.
Futurity and the subjunctive are not the same. A future tense, either a true one as in Latin or a periphrastic one, expresses something that will likely happen in the future. The subjunctive expresses conditions contrary to fact, conditional statements, exhortations and demands, etc. In languages with a non-defective subjunctive and a future tense, a verb can be both future and subjunctive (e.g., expressing a future state that is conditional or unlikely). Latin has a future subjunctive, but it is archaic and exceedingly rare by the classical period, and periphrastic methods are the more common way to express it. English doesn’t have a future subjunctive because it has no true future tense and its subjunctive is defective (i.e., incomplete).
In general, tense is an inflection on a verb used to mark the time when the action occurs. The mood, while expressed through the verb, expresses an emotion or attitude that influences the entire sentence. And the voice, passive or active, expresses the relationship the subject and object have with the verb.