Translation:If it were impossible, it would not be possible.
Yes you are missing a grammatical rule; but it's not uncommon any more. It's called the "subjunctive" mood which is used for hypothetical statements, (and some other conditions not needed to go into here). The correct way to say a hypothetical about whether or not you would do something... is to say: "If I were king...then...." or "If she were to talk to me that way, I would ....." were is the correct verb to use here and not was, although with passage of much more time, the fading subjunctive may be used less and less; but until such time, yes, the correct answer is "If it were possilbe" and definitely not "if it was..." Hope that helped.
If you want more on the subjunctive in English, open this link (in another window or tab so you don't lose this page).