"The cat eats the frog."
Translation:Kedi kurbağayı yer.
Because "kedi" is nominative, "kurbağa" is accusative because it is the object of the verb. Only accusative nouns take the -(y)I suffix.
In this sentence, "kedi" could be "a cat" and "the cat". If you wanted to emphasize that it's "A cat" you could say "bir kedi kurbağayı yer" (not sure about the last part though)
An interesting fact. The Old Turkic people thought that frogs and turtles were the same creature with a small difference; turtles were frogs with armours, so, they called them as "Kaplı Bağa" [(a) Covered/Aemoured Frog] or "Tos bağa" (Tos is kind of a noise when something hits another tbing, they probably gave that name because of the noise when they hit a turtle, haha, cruel), and a frog was "Kuru bağa" [(a) Dry Frog] or simply "Bağa" [(a) Frog]. So, in Modern Turkish, we say Kaplumbağa/Tosbağa and Kurbağa. The words Tosbağa and Bağa are relatively archaic.
They dont use logic but i feel like they are ment for fun you should never dout the possiblities you might need to say or read these sentences duolingo gives us we are trying to learn the language of turkish so we must know every word or sentence no matter what in order to speak it completly (ex: you are reading a book in turkish to a kid and it says the dog is reading a news paper there is no logic bacause its a fairy tail but you need to know how to read it because you are reading it to a child) i hope you understand after this explanation! :)
"kaplumbağa" and "kurbağa" are simple words; "bağa" doesn't mean anything in either of them.
Nişanyan thinks otherwise; in his etymological dictionary of Turkish, he writes that bağa comes from an Old Turkish word baka meaning "frog", and that kurbağa probably comes from kuru baka and kaplumbağa from kaplı bağa (= örtülü kurbağa, kabuklu kurbağa: covered frog).