I think it's proper grammar to put an indefinite article "a" or "the" before "duck", and "a" before "dog".
e.g. "The duck is not a dog"; "A duck is not a dog".
I'm not sure if "Ducks are not dogs" should be a proper translation, because the nouns in the sentence are singular (even if it conveys the same idea).
Because it makes no sence - it's ungrammatical. After the verb 'być' (to be), one has to use the instrumental case, which in this example is 'psem' ('pies' being the nominative).
Trying to explain it to English speaker... mhmm, imagine a sentence, "Jestem psem" - 'I am a dog'. 'Jestem pies' would sound like 'I be dog', ears begin to bleed.
this example is nonsense.
"i be dog" = "ja być pes."
all you changed was the be-verb. not the case. here's how it works in slavic languages.
czech: Jsem pes. (NOMINATIVE)
russian: я собака (NOMINATIVE)
serbian: Ја сам пас. (NOMINATIVE)
polish: Jestem pesem. (INSTRUMENTAL)
each slavic language has its own case rules and exceptions DETERMINED BY VERBS AND/OR PREPOSITIONS. in this case, the verb "być" demands the instrumental case is used.
when you get better at the language, you can find lists of verbs and their associated cases to practice with. But for now...ya just gotta suck it up.
Great comment! In your Russian example, the present tense verb есть is omitted, but in past or future tense, instrumental can be used: Я был собакой/псом, Я буду собакой/псом. Also, the verb являться is used sometimes in the present tense and does use the instrumental case in the object. Это является собакой/псом.
Yes. Staying with affirmative sentence, you can have:
X to Y (Y in Nominative)
or X jest Y (Y in Instrumental).
Also, when negating an "X to Y" sentence, you put "nie" before the noun. There kinda is a hidden "jest" in such a sentence: "Kaczka to (jest) pies", "Kaczka to nie (jest) pies". Using this 'jest' is not wrong, but rather not very natural.
Kaczka nie jest psem means "A duck is not [in the state of being] a dog," which is correct. If you say Kaczka nie jest pies, it's like saying "Duck no is dog." --broken English. Instrumental case means "in the state of being" when following the verb być: jestem, jesteś, jest, etc.
Depends on the contects :D violin = skrzypce. "Gram na skrzypcach" - I'm playing the violin (locative). "Nie ma skrzypiec" - There's no violin (genitive). "Flet gra ze skrzypcami" - The flute plays with the violin (like on a concerto - instrumental). You can multiply the examples and find a perfect use for each case :)