Me trying to think if I've written the right answer on duolingo.
This is confusing because in the previous lesson we learned the subject endings change to indicate groupings, and that High Valyrian does not use "and."
You're half right. HV does use "and" sometimes because in a phrase like this one with no subjects to modify you need a conjunction.
Well Issa se daor.
Take your upvote
It's correct for nouns, the second subject ending becomes prolonged. Yes and No aren't nouns and they can't be modified.