Ohh I realize now :), Katterna means "the cats", but can i just say "Katter äter möss", as I just saw "Ankor äter fisk"?, I know it means "ducks eat fish" (without the "the"), it's just that when i translate it to spanish both for me means "the" ducks, i cannot say "patos comen pescado", I always say "los patos comen pescado". A Little bit confuse for me, but I try :)
This is an exercise just to let you prove you can tell "cats" (katter) from "the cats" (katterna). Time will come to talk as in the real world. For now, let's keep practising...
It's indefinite, and has to be translated as such. Here using it as indefinite implies they eat mice in general, not some specifically unfortunate ones.
I don't understand why I can't say "mouses". I'm not english native and for me "möss" (plural of "mus") means "mouses" in english