"A bad duck is eating a fish."
Translation:Zła kaczka je rybę.
Because you need to use Accusative Case here. Ryba- is a feminine gender. In Accusative Case the ending is -ę for such nouns.
May I recommend installing SwiftKey, then you can have up to 5 different languages on one keyboard, with all the diacritics. And then of course you just have to press a 'normal letter' for a longer time to see the diacritics.
Well, first you need to change your PC keyboard to Polish, and then it's either left ctrl+left alt+the 'normal' letter; or right alt+the 'normal' letter.
Ż is under Z, Ź is under X.