I had always understood the subjunctive mood to mean a hypothetical present tense. However, I think it might also imply that the hypothetical event is outside the realm of possibility. Is that necessarily the case?
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I don't think there is any implication of impossibility per se, although the fact that the statement is hypothetical might bias its use toward the impossible, e.g. "if I were you I'd buy the cheaper brand."
In constrast consider "if I were to get one I'd buy the cheaper brand." There is no indication that I won't "get one", in fact I might be seriously considering it.
[EDIT] Anicul adds:
The first sentence is an example of a present contractual conditional statement and the second is a future less vivid conditional statement. Present and Past contractual conditionals are contrary to fact, while future less vivid conditional statements imply uncertain potential for action.
Traditional grammar, following the Latin model, regards the subjunctive as an inflectional category. This is understandable in a language such as French which has a full set of inflections for the present and perfect subjunctive, many of which serve no other purpose. English, on the other hand, uses only the plain form of the verb to indicate the subjunctive. The authors of ‘The Cambridge Grammar of the English Language’, Rodney Huddleston and Geoffrey Pullum, and Bas Aarts in his ‘Oxford Modern English Grammar’, regard the subjunctive not as an inflectional category, but as a syntactic construction. That is to say, it is identified not by its form, but by its function.
Let’s take as an example the sentence We demand that he resign. It is used here as part of a mandative construction, that is, one that, in Huddleston and Pullum’s words ‘includes a component of meaning comparable to that expressed by the modal verb must’. Subjunctive resign is identical to the form of the verb used as the infinitive and as the present tense in all persons except the third person singular. However, in such sentences the indicative is also available and many native speakers, particularly native speakers of British English, will choose it and say, in all but the most formal contexts, We demand that he resigns. In truth, the subjunctive is rare in contemporary English, and may well disappear altogether over the next 50 to 100 years.
There seems to be a popular belief that conditional sentences contain subjunctives. Other than in one special case (and even there it’s doubtful), they do not. The three principal types of English conditions are:
These use the modal verbs will and would in the main clause. Will typically expresses, prediction (as here) and, secondarily, volition. Would typically expresses, primarily, unreal meaning (as here) and, secondarily, habit, volition and prediction.
The special case arises in a sentence such as If I were you, I’d have a haircut. Many grammarians still regard this use of were as subjunctive. Huddleston and Pullum do not. They call it irrealis were. They explain:
Subjunctive mood means future 'I would if I could'. All other uses are derivatives. 'I should know = I better know' &c. In conditional sentences it serves to mark hypothetical future, 'I wish I could'.
The need for present subjunctive didn't suddenly disappear over the past two centuries. I'd say we still use present subjunctive frequently today, only replacing Shakespeare's be with present, not past subjunctive (so-called "irrealis") were.
This is clearly present, not past tense usage. I consider replacement of be with were for present subjunctive an example of beneficial evolution of language, in that it simplifies without impairing clarity or understanding. (Shakespeare's if this be would today be understood as indicative, no different from if this is, just as Though this be madness, yet there is method in 't.)
Of course, were can still be used as past subjunctive as well, but it's usually replaced by other moods/forms, such as had been.