Since we're playing grammar here, the subjunctive is a mood and not a tense. Grammatical mood is a form of verb typification. Syntactically, mood is denoted by verb inflection, which is what caused you to notice with such dismay that people have been using the subjunctive incorrectly.
In English there is only one fully operational mood--the indicative--and so I suppose few schools bother to teach the concept at all. I myself only learned about it in third-year high school Spanish. The handful of other moods which do still exist in modern English are either vestiges or special cases, and either way they not functional moods and are usually taught as irregularities in the language without the word "mood" ever being mentioned.
The subjunctive mood indicates a statement whose action has the condition of being hypothetical. If you were omniscient, you would have already known that. But you're not omniscient; I was merely speculating; and this was expressed by my last sentence, via the subjunctive mood.
Another way of looking at mood is that, from the English perspective, nearly the entire function of mood is accomplished by the indicative, without verb inflection excepting only a few specific moods. Indeed, from the English perspective, the concept itself of mood is somewhat vague, and a list of moods would look mostly arbitrary and sometimes repetitious.
My talents do not extend to a functional understanding of other languages (besides Spanish), but you might look to Legend of the Past for an insight into how Hebrew handles mood, or to Daniel Krispin for Latin and Greek. No doubt these individuals, being fluent in the respective tongues, will be able to offer the sort of enlightenment that I as a speaker of your own native language cannot provide.
