And everything I know about Polish grammar comes from Russian class. I've never taken a Polish language class in my life, whether for foreigners or native speakers. In retrospect the funniest part of Russian class was learning about verb aspect, which is both one of the most fundamental parts of Polish (and Russian) grammar, and generally the hardest thing for (non Slav) foreigners to get - I had no idea that aspect existed even though I'd been using it correctly my entire life.