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 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.
You can tell the difference between "she" and "they" from the verb.
The verb for sie "she" usually ends in -t (e.g. sie isst "she eats, she is eating"), while the verb for sie "they" usually ends in -en (e.g. sie essen "they eat, they are eating").
(What's harder is the difference between "her" and "them", i.e. when sie is an object. That's impossible to tell without context.)
In the fill-in-the-blank exercise that I can see, the options are sie isst - sie esse - sie esst.
"she eats" is sie isst, "they eat" is sie essen.
So from the options isst - esse - esst, only isst can be correct after sie.
sie esst is not correct because esst is the verb form for ihr.
sie esse is not correct because esse is the verb form for ich. (Leaving aside the subjunctive for now....)
From context. Even in English, if you use "they" and "she" in the same sentence, the rest of the sentence will change. "She is dancing" "They are dancing" "She does the macarena" "They do the macarena" "She takes a bow" "They take a bow"
So even if the words were the same, or even removed entirely, if I gave you sentences like "_ is now famous for the macarena" "_ are now famous for the macarena" I would hazard that you could still fill in the correct words. So in German, in a sentence like "Sie trinkt" "Sie trinken"
Once you get the context, you should be able to fill in the correct word right away! Hope this helps!
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).