For whatever reason, somebody once thought that it was necessary to treat a verb completely differently if what you were talking about was just supposin’.  Ya know, nothin’ serious an’ all... jus’ supposin’.  Apparently they wanted to make darn sure that the other person would have no trouble knowing that when you were talking about making a time machine with your toaster... yeah.  You weren’t really being serious.  In other languages, they just tacked on extra letters to a verb and called it subjunctive, but, really, that’s just a whole lot of work to come up with a different way to say “was.”  I mean, geez, aren’t there enough ways already?  So in English, we just used the past tense.  Therefore, instead of saying, “I wish I was...” which makes sense, you’re supposed to say, “I wish I were” and “If I were” and “Supposing I were” and you know what?  It’s just not that hard to know that plans to drive your Chrysler to Japan... yeah, you’re just supposin’.    I mean, if you can’t figure out that my blender is not really self-aware, as it is, I don’t think extra vocabulary is going to help.