"A duck is not a dog."
Translation:Kaczka nie jest psem.
This is how I see it, but I am not sure this is the best (correct?) explanation... Let us skip "kaczka" to make it simpler. Consider the following four sentences.
"It is barking. It is a dog." = "Szczeka. To pies"/"Szczeka. Jest psem"
"This is a dog" = "To jest pies" (e.g. when pointing at a dog)
"This is a not dog" = "To nie jest pies" (e.g. when pointing at a cat)
"It is not barking. It is not a dog." = "Nie szczeka. To nie pies"/"Nie szczeka. Nie jest psem"
So we have the following grammaticaly correct sentences "To pies. To jest pies. To nie jest pies. To nie pies." (they are not equivalent in meaning (and the precise meaning could depend on the context/intonation/etc.) and the role of the word "to" varies in those sentences). Basically "nie" comes before the verb ("jest") and the verb comes after "to". When using all three "nie" is after "to", but before "jest" (unless you want to say "Nie, to jest pies" = "No, this is a dog" ;P). When you skip the verb ("jest"), the order does not change.
Is this helpful? Does somebody have more insight?
PS: You could use also "Kaczka to nie jest pies", but again the meaning would be a little bit different (more emphasis) from "Kaczka to nie pies"/"Kaczka nie jest psem".
In Polish we have 7 cases. I'll try to explain them using word "kot" (a cat).
"Mianownik" (Nominative) - Main case, used when we are naming something. Subject of the sentence. "Jego kot jest czarny" - His cat is black. "Koty liżą masło" - Cats lick butter.
"Dopełniacz" (Genitive) - Case with really wide usage. 1) Describing possesion, 2) Using negations, "To nie jest wina kotów" - That's not the cats’ fault. 3) With some verbs. "To zabawka jej kota" - This is her cat's toy.
"Celownik" (Dative) - Case used when describing aim of the activity, or for indirect objects. "Daję jeść kotu" - I'm feeding a cat. "Pomagamy kotom" - We are helping cats.
"Biernik" (Accusative) - Case used for direct objects. "Znalazłem kota" - I found a cat. "Widzę koty" - I see cats.
"Narzędnik" (Instrumental) - If something is an instrument of an activity, way which led for result, you should use that case. "Idę z kotem do weterynarza" - I'm going to the vet with my cat. "Psy nie lubią się z kotami" - Dogs and cats don't like each other.
"Miejscownik" (Locative) - Always with preposition. Used for describing location of an action, or when you are thinking about something. "Myślę o kocie" - I'm thinking about a cat. "Anioły żyją w kotach" - Angels are living in cats.
"Wołacz" (Vocative) - Very rare case. Used only for calling something. "O kocie, czemu to zrobiłeś" - Oh cat, why have you done that. "O wielkie koty" - Oh grand cats.
Many people go a lot too far with the negation and Genitive thing. The rule isn't "negation = Genitive".
The rule is "negated Accusative = Genitive". And because Accusative is the most common case, and the basic one, people make such mistake of treating every negation as Genitive. But every other case than Accusative, when negated, stays the same anyway.
It is just an amusing (or confusing) coincidence that in the construction using the Polish "TO" the expected nominative is "pies" and your incorrect answer "psa" is regarded as the "one letter typo" of the potentially correct noun: "psy", that is why it is... accepted. (I think it is an error since your mistake has more than one letter). To correct the sentence in a way it makes sense and still uses the nominative, you have to change the singular subject "kaczka" to the plural "kaczki" to balance the plural nominative "psy" (potentially correct "typo"):
A duck is not a dog - Kaczka TO nie (jest) pies
Ducks are not dogs - Kaczki TO nie (są) psy
I have checked the incorrect option "Kaczka to nie psa", and... it is
not accepted now and the correct suggestion is "Kaczka to nie pies".
But I am happy to notice the answer in plural: "Kaczki to nie psy" is accepted here, because, in fact, it really has the same meaning as: "Kaczka to nie pies".
There are two options, so my guess is that your mistake was mixing them. Or perhaps you made some typo and the algorithm corrected you to the other version, although if it still works how it used to, then it should show the version with Instrumental.
"Kaczka nie jest psem" has "dog" in Instrumental.
"Kaczka to nie pies" has "dog" in Nominative.
https://forum.duolingo.com/comment/16373167 - more information here.