Translation:The apple is food.
it doesn't accept an "an". first of all, when a language one is learning doesn't use articles, then translation to english should not enforce usage of particles either. we're not learning english here, so stop molesting us with english grammar. secondly, if you're gonna force us to type articles, then clarify which article should be used, no one here is a mind reader.
The problem with Duolingo, if it's a problem. It's that they don't teach grammar here, and many people don't understand that, you can't come here without knowing what is a particle, the order of the sentences, etc, or else you'll get frustrated.
I'm not telling you to stop taking the course, but to complement duolingo with a grammar course it would be great for you leaening process. There's an amazing page that teach korean grammar for free: https://www.howtostudykorean.com/
Go to your web browser and search for Duolingo, but make sure you use the web version instead of the app ,as they may try to remind you that you have the app if you do. Click on Learn at the top of the page then click on a lesson, then click on Tips. If you are asked to sign in, you can use the same username and password to login.
The first time you talk about something you would use the first. It is the subject. “The apple is food.” Next time since you already mentioned it, you would use the second. “As for the apple, it is food.” Scroll up for a site that will help you with that and further down for links to the tips and notes.
And now you must click on Tips at each lesson.
Well, the Tips and notes cover that, but here is more information:
No the Korean verb is at the end of the sentence.
That is a particle that marks the word it ends as the subject.