Are the "us" and "them" pronouns in different cases? If I hover over them it says "nas" is genitive or accusative whereas "nich" is locative. But I can't figure out why they should be different.
It looks like the dictionary hints have been updated: „nich” is both locative and genitive, and this use is genitive „nich” I think.
Well, I'm afraid you're wrong here. While Locative is mostly about location (its Polish name, Miejscownik, also keeps this connotation), when we learn cases at school, the basic questions of Locative are not in fact connected with location. The basic questions are: o kim? o czym? (about whom? about what?) And this is exactly what happens here. For both of them. I'm afraid the hover tips are still very imperfect...
So sentences containing the form „o [someone/thing]” always put that object in locative case? I'm still very unsure about which cases are used when, but I've been noticing that „o” often takes the locative, but not always.