The key to the subjunctive is that the subjunctive is contrary to reality. This is not about lies (lies pretend to be reality), but about correctly describing something that isn’t. The gradeschooler in the first panel of this Frazz gets it right:
She even says it’s a wish. Don’t say something like, “I wish there was…” when you aren’t referring to something that doesn’t exist.
On a side note, I’ll add that the Greeks had a mood even stronger than the subjunctive, used only for wishes. It’s called the optative. We have to make the subjunctive work for both wishes and contrary-to-fact things.
Need a non-wish subjunctive: How about “He would go on vacation, but he’s too busy.”