Translation:I eat an apple and she eats bread.
This is the accusative case - the apple is the direct object of the verb to eat. In German, the masculine changes in the accusative. If it were, I am eating the apple, it would be Ich esse den Apfel.
The other genders don't change in accusative, but they ALL change in the dative (indirect object - it's tons of fun!). Pronouns change also depending on case.
I do not think that you need to be so confused. Yes, Duolingo could have chosen another sentence with '... sie isst ein Brot' or '... das Brot'. However, that's not the case. It does not matter; the sentence still works without article. Apparantly, Duo thinks that she just eats some part of a bread or perhaps several breads, e.g. buns.
I suppose it is the same as we have in Norway - bread can be referred to as a single bread, or as an undefined amount of bread. In the last case, you just say Brot. Take for example if you are at a restaurant and get served several baskets of sliced bread. It would not make sence to refer to it as one or two breads, it is just a lot of it.
I think German might be like English in that we do not say, "She is eating a bread." She could eat a slice of bread or a loaf of bread, but not "a bread". On the other hand, we say, "I eat an apple," not, "I eat apple." I could eat apples, but not "apple" by itself. I don't know that there is a rule for this (although with English, there probably is a rule and these are the exceptions!). Actually, since I have been thinking about it, this probably has to do with singular nouns and plural nouns--bread is a plural noun just like apples. There isn't really a singular noun for bread in English, but apple is a singular noun. So that might be the explanation--'a'/'ein' wouldn't apply because bread is plural (a/ein indicates one, not more than one, but since we are speaking in general rather than in specific, we don't need to use 'some' with 'bread').
I recommend moving away from the idea that you can translate verb forms exactly between English and German. Instead, you should think about them in combination with who or what it applies to.
If you only look at the example of "I eat" = ich esse, maybe you think that esse = "eat". But then you run into the example of "we eat" = wir essen. Now essen also means "eat"! This gets confusing - so try to learn how the verb changes to fit the person/thing. This is called conjugation.
By the verb form -- sie "she" almost always has a verb form ending in -t, sie "they" almost always one ending in -en.
sie isst = she eats, sie essen = they eat.
On the other hand, it's impossible to tell the difference between sie "her" and sie "them" when they are the direct object, if there is no context. Ich esse sie. could mean either "I eat her" or "I eat them", for example.
The short answer is:
- ein Apfel when it's the subject (the thing that is or does something)
- einen Apfel when it's the direct object (the thing that gets something done to it)
In this sentence, it's the direct object of the verb essen -- it's the thing that gets eaten.
And note that Apfel is always capitalised -- it's a noun, after all.
Why can't it be "I am eating... and she is eating..."
That looks as if it may be a valid translation.
Please show us a screenshot where we can see exactly what kind of question you had and what your exact answer was (upload it to a website somewhere and tell us the URL).
I have no idea why I am wrong
Nor does anybody else. We cannot see what you wrote unless you show us.
Can you guys tell me why?
If you have a screenshot that shows the exercise, your entire answer, and Duolingo's reaction, then perhaps.
Upload your screenshot to a website somewhere, please (e.g. imgur) and tell us the URL of the image.
Otherwise nobody can guess what might have happened.