"The crab is an animal."
Translation:Krab jest zwierzęciem.
One is Nominative (and Accusative as well), the other is Instrumental.
For example, this sentence could be translated with two versions: "Krab to zwierzę" and "Krab jest zwierzęciem".
Is there a difference on how both would be perceived, or is it just preference on which is used, in this case?
My personal view is that "Krab to zwierzę" is more like "A crab = an animal" and "Krab jest zwierzęciem" is more... descriptive. In most cases the Instrumental form sounds superior to me, although a moment ago I was commenting on "This animal is a tiger" and I'd use "To zwierzę to tygrys" there... because "This animal = a tiger" makes more sense there.
I wrote "krab to zwierzęciem" and it marked it as wrong. But it shouldn't be wrong, correct?
It's wrong. You mixed two constructions, "Krab to zwierzę" and "Krab jest zwierzęciem".
The Instrumental form of "zwierzę" is "zwierzęciem".
The word "zwierz" exists and its Instrumental would indeed be "zwierzem", but that's a rather dated, a bit poetic (?) word and usually denotes some large, wild animal.