łosoś becomes łososia why?1 answer
Hi Ania, it is difficult to say, it is just ... how it is.
I think the best way to learn Polish cases it to start with the first two Mianownik (nominative) and Dopełniacz (genitive) and then build on that. It is worth remembering that the last case, Wołacz (vocative), is not used so often and that for many nouns it is the same as Mianownik.
Find some information online about these 3 cases and try to look for them in the sentences. A great website where you can check correct cases is odmiana.net.
For this particular word łosoś salmon, the Polish cases are here:
Once you know the usage of particular case and learn how to form it, move to the next case.