"He eats an orange."
Translation:Er isst eine Orange.
The object is not feminine.
But the word for it, Orange, is feminine.
Noun gender is pretty arbitrary. There's no "reason" why Gabel ("fork") is feminine, Löffel ("spoon") is masculine, or Messer ("knife") is masculine.
Words themselves are pretty arbitrary. Tables have four legs, but the word "table" does not have any legs -- no little lines reaching below the line of writing as in "p q".
I think there's two differences, one is fressen and frisst are both used to usually go along with talking about animals eating, or if someone eats in an unpleasant of fast way (like an animal) frisst or fressen may be used, while esst and essen are correctly used when talking about people. I believe there may be another difference involving the words er/sie in context, but I'm not sure. Hope this helps.
Why orange is sometimes eine and other times einen. One sentence i wrote einen, and correct answer was eine, so next sentence i remembered that and wrote eine, but then the correct answer was einen! This drives me crazy and i feel like it is impossible to learn these.