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?
Learn English – the subjunctive mood
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You're confusing traditional Latin grammar terminology with English grammar terminology,
and with modern linguistic terminology, as well.
Mood, Voice, and Tense were traditional inflectional categories of Latin verbs. I.e,
every verb in Latin was inflected (marked uniquely) for some mix of mood, voice, and tense.
Latin had six tenses (by a strange coincidence the same six you listed),
four moods (indicative, subjunctive, imperative, and interrogative),
and two voices (active and passive). That was Latin.
English has two tenses (Present and Past), no moods, and no voices.
In particular, English has no subjunctive mood, so you don't have to worry about it any more.
However, many other languages have rich inflectional systems, even richer than Latin.
Sanskrit and Greek both had a Middle Voice as well as Active and Passive, for instance,
and an Optative Mood (used for things one wishes and hopes for), and Sanskrit also had a Benedictive Mood (used for blessings).
And that's just Indo-European. There are lots of other ways to organize these matters.
It's important here to distinguish between the preterite form of a verb, and the past tense meaning that is normally associated with the preterite form. The modal preterite use of some English verbs indicates modality, as the OP points out (not past time). For example, in
I wish you went to the doctor more often than you do.
...or..
If you went to the doctor tomorrow, I'd drive you.
...use of the preterite form of the verb (went) does not indicate past time.
To answer your question (why you can't say "If I were you, I went to the doctor"), English does not grammaticalize a generic concept of modality (nor does it have a robust grammatical category "subjunctive" as French does), but instead has several common constructions that can be used to convey different types of modality. The modal preterite construction cannot be used in the same situations as a modal auxiliary can. The modal preterite expresses modal remoteness, generally in one of the two types of constructions:
- Conditional constructions. (e.g., If she injured herself, I'd rush to help.)
- Expressing a counterfactual situation, but only in subordinate clauses which complement specific verbs that have an element of uncertainty in their meaning (e.g., wish, think).
Auxiliary verbs can be used to convey a larger range of modal meanings (with different auxiliaries having different modal meanings associated), some of which include remoteness. In a sentence like "I would go to the doctor (If I were you)," would expresses two types of modality: propensity (lexically) and remoteness (preterite form).
Best Answer
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.