Interesting. Do you think that Polish and Russian differs a lot overall though? (I don't speak any Russian so I am just wondering.)
no, I think they are pretty similar, but there are some "false friends" like owoc, dywan, portfel, and zapomnić (this means "to remember" in Russian)
also gruby (rude in Russian) pukać (fart) niedziela (week) żadny (greedy) pokój (peace) kraj (district) pytanie (food) etc, etc
Is zdanie more of a carefully constructed opinion that opinia? I am a russian speaker and as noted elsewhere in the comments, здание means building, so it got me wondering.
Mostly, "zdanie" is just a sentence. Just like the previous one. And this one. Any sentence.
But you're right, it can also be "moje zdanie", which I guess isn't really different from "moja opinia".