"The elephants eat an apple."
Translation:Filler bir elma yer.
Actually, it happens sometimes.
The "not definite → not accusative" rule only works if the object is in the immediate vicinity of its verb:
Ex: Beş ayda ancak bir kitap bitirebilirim. (I can only finish one book in 5 months).
Let's change the place of the "bir kitap"
"Bir kitap ancak beş ayda bitirebilirim" is no longer acceptable. You have to use the accusative:
"Bir kitabı ancak beş ayda bitirebilirim."
Because 'filler' is not the object of the sentence. We put the accusative suffix when we say about a specific direct object. E.g.: Aslanlar filleri yer. = The lions eat the elephants. 'Filleri bir elma yer' is not correct because 'filleri' is the subject of the sentence and it's in the nominative case.
I hope I can help you. :)
"The elephants eat an apple." Translation: Filler bir elma yer.
Why "elmayi yer" is wrong
There is no "the" in the English question for the object. You do not need to use the accusative case suffix & your answer will be rejected by Duo. Apple is the indefinite, direct object in the nominative case. It can only go before the verb. There is "an" which equates to "bir" in Turkish & "one."
These answers are accepted by Duo:
Filler bir elma yer. - The elephants eat one apple.
Filler bir elma yer. - The elephants eat an apple.
I got this answer wrong last year as well.