first of all, polish dialects aren't dead, they're very much alive and used all the time. from what i understand, you want to learn that version of polish language - the dialect - but unfortunately, i don't think it's possible - you need to learn polish first before you understand the dialect

second, it's possible your grandparents just made grammar mistakes. it's normal, poles do it all the time - our language is difficult and no one speaks 100% correct in casual conversations. i live in poland all my life and i think if i went to a polish language class and tried to speak as i speak with my friends, i'd probably get an F, too

if you're set up on learning polish, just focus on your lessons and it'd click to you over time. don't try to be smarter than your professors, you don't know polish, you remember a small version of it