[euyyn]

If you don't quite remember, or are unsure about, their preferred pronoun, it doesn't matter. Really, don't sweat it.

What we're trying to tell you here isn't "you should concentrate harder, learn it by rote, and never ever say it wrong". Rather: "just give it as much thought as you'd give someone telling you to call them Dave." No more, because nobody wants to make your life any harder, and any sane person would understand it if you forget. This is a preference, not an imposition.

And also no less, because at the end of the day it's a very little thing for you, and might mean a lot to them.

Grammar and plurals become weird? - You've already given it more thought than I think I'd have :) In my case, because I'm not a native speaker, I would have probably messed the grammar anyway.