"The horse eats an apple."
Translation:At bir elma yer.
Because "an apple" is a general direct object and doesn't need the accusative ending (whereas "the apple" would refer to a specific apple, and would need to be in the accusative). It's a little confusing, but there's more information and some examples in the "tips and notes" of the Food and Accusative skills.
I read in another comment thread how "bir" can be excluded in the subject, but not with the object, because that would mean it was plural.
If that's true then "At elma yer" would not be a correct translation here, right? That would mean "The horse eats the apples", correct?