In formal conversation it is not unusual, you can say bir büyük elma or büyük bir elma no difference. But in official conversation büyük bir elma is much better
It should be counted right. Your logic is right, but Duolingo wants it to be an article (I don't know why). If you do the same mistake, report and say that my answer should be excepted.
sen yersin: you eat. in english we use the same verbs for all pronouns, so i explain this in french for you. sen yersin; tu manges, sen; tu mang; yer , sin;es.
No you don't use the same verb for all pronouns in English, right? I eat, you eat, he eats. I mean there are minimal differences but there are still differences