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 :)
Virtually no native English speaker knows where this came from or can explain it ! (You are in good company.) For fun, we can say mouses have houses and everyone knows. Saying mouses instead of mice--no biggie.
Now, lice instead of louse, that's a problem!
But why not spice for plural spouses? Illegal !