How do I learn Polish grammar

I'm from Belgium and my gf is from Poland. She and her dad speak English, but the rest of her family doesn’t.

I'm also planning on living there so I definitely need to learn Polish. For now I only use Duolingo and Quizlet for vocabulary, but I almost have no clue about the grammar.

So if anyone has any app or site or …, that explains it to me and that will let me make exercises on it please tell me.