"The girl eats the orange."
Translation:Kız portakalı yer.
Kız is the subject so it cannot be in the accusative. The way you wrote it would have kız in the accusative. If you're the one writing and you want to differentiate if the subject is "a girl" or "the girl" you can write "bir kız" to show that it is "a girl" but if it is just "kız" (as the subject) then it can be either "the girl" or "a girl"
I'm not expert on Turkish, I'm Indonesian, but in my school Turkish is learned
They use ı when the last vowel is a/ı Masa-sı, Dolap p become b Dolabı, Yağı, Kapı-sı, Duvar-ı
They use i when the last vowel is e/i E.g. Başkent-i , Ev-i,
If the last letter is e/i, you have to use "si" Eg. Nere-si? Sandalye-si, Pencere-si