"You see a big animal."
Translation:Widzisz duże zwierzę.
Which "you" is being used here, addressing a single person or a group? I'm still having trouble with the suffices, but i think widzimy and widzicie covered both cases, but I'm probably wrong.
You can be both widzisz (sing) and widzicie (plu). Without the context both are correct.
Widzimy is 1 person plural - My widzimy - we see.
Why if it's a dog or a cat it changes it's case (for example Piec to psa) but if it's 'animal' the word stays the same?
because kot , and pies are masculine animated nouns, and zwierzę is neuter noun.
Masculine nouns (in singular), can change in accusative case or not depending of their "animacy" (animated nouns usually have accusative=genitive, not animated nouns have accusative=nominative)
Neuter nouns have always have accusative=nominative.
Feminine nouns (in singular), have accusative form that is different form other cases.
Can't you also say "Widzisz wielkie zwierzę"? Or is "duże" simply a better sounding term for big in this context?
"Wielki" is bigger than "duży" (so I guess 'large' would come in handy here), but there aren't any objective norms saying when duży ends and wielki starts, plus the English wiktionary gives both 'big' and 'large' as translations of 'wielki' in terms of size.
did you write "ty widzisz duże zwierzę"? this is correct and should be accepted. Maybe you made some other mistake.