Heck, I'm an English-only speaker, and I still get were/was wrong all the time. Honestly, I have no idea what the rule is there. They both sound correct-ish to me.
English grammar is often confusing in cases like this because verbs mostly don’t have distinct spellings/pronunciations for the subjunctive mood, even though the subjunctive conjugations are part of the grammar.
It also doesn’t help that “subjunctive” is kind of the “else” condition among the three verb moods, the other two being “indicative” and “imperative,” so you often notice that it’s the subjunctive by ruling out the other two.