Maybe it is right example: cocoa (in Polish - kakao) http://pl.wiktionary.org/wiki/kakao#pl The second example is: Mrs dentist (pani stomatolog) who? - pani stomatolog, whose? - pani stomatolog, at whom (I'm looking, staring) - na panią stomatolog, whom (I see) - panią stomatolog, with whom (I walk) - z panią stomatolog, about whom (I say) - o pani stomatolog, There are many more examples but these two seem to me enough. - http://pl.wiktionary.org/wiki/Kategoria:Nieodmienne_rzeczowniki_w_j%C4%99zyku_polskim - Greetings. Jeśli coś pomyliłem, to przepraszam i proszę o korektę.

Olá! Cześć! I think that Polish is constantly changing and evolving as other languages. But still, there are many words - nouns and verbs, which are the same in our language since the medieval time: słońce - the sun, księżyc - the moon, water - woda and many more. I'm not sure if you mean such type of change or you had in mind verb conjugation and grammatical case. I know that many foreigners have difficulties especially with the last issue. Here you have short example: A city - Warszawa, a colour - zielony (green) Mianownik (kto? co?): Warszawa, zielony Dopełniacz (kogo? czego?): Nie widzę Warszawy, zielonego Celownik (komu? czemu?): Przygladam się Warszawie, zielonemu Biernik (kogo? co?): Lubię Warszawę, zielony Narzędnik (z kim? z czym?): Idę z Warszawą, z zielonym Miejscownik (o kim? o czym?): Dyskutuję o Warszawie, o zielonym Wołacz: Hej Warszawo!, zielony! Hope that helps a little. If you have more doubts, ask me. Powodzenia/Good luck!

I am afraid that all words are changing.