In English the "a" can be replaced with one without changing the meaning.' She eats one apple' and 'she eats an apple' mean essentially the same thing, it's just when you want to put emphasis on 'one'. You can put emphasis with which word you stress in german. 'Sie isst einen Apfel' 'Sie ist EINEN Apfel'
Eine Frau = a woman
When a woman (eine Frau) is the subject, the verb gets conjugated the same as though he/she/it (er/sie/es) was the subject.
The conjugation of the verb essen for er/sie/es is "isst".
"Esst" is the conjugation for the subject "ihr" (you, informal, plural).
See this conjugation chart:
You can't if you're looking at the verb only. The subject makes the difference. If there's a "du" then it's 2nd person singular, otherwise it's 3rd person singular. There are more verbs like this: vergessen, *fressen, hassen, *lassen, prassen, *fassen, *passen, hissen, *missen, *pissen (vulgar!), *beißen, *heißen, *reißen, *scheißen (vulgar!), spleißen, *gießen, *schießen, *schließen, genießen, niesen, vermiesen, *reisen, *preisen, *weisen. But not: wissen, müssen (they're irregular: du weißt, er/sie weiß etc.) Does anybody have more of these?
The word Apfel is grammatically masculine.
It's also the direct object of the verb "eat", so it has to be in the accusative case.
The masculine accusative form of ein is einen.
That is why it is einen Apfel.
eine would be feminine -- for example, if she were eating a tomato (eine Tomate).
For a neuter noun, it would be ein -- for example, if she were eating a horse (ein Pferd).
Instead of "a apple", you should write "an apple".
Words that start with a vowel sound (such as "apple") take the indefinite article "an" in English.
Unfortunately, Duo's corrections are not always as good as a human's in terms of finding "what the learner probably meant".
When the apple is doing something (Der/ein Apfel fällt - The apple falls) then you use der (the) or ein (a/an). When something is being done to the apple (Er isst den/einen Apfel He eats/is eating the/an apple) then you use den/einen. When words do different things in a sentence (acting, being acted on, receiving actions, owning things) they are said to be in different "cases" in German.
The man eats (The man is doing the action: Nominative Case - Der Mann. Der Mann isst.
I eat the man (the man is being acted on: Accusative Case - den Mann . Ich esse den Mann.
I gave it to the man (the man is the indirect recipient of whatever "it" is: Dative Case - dem Mann) Ich gab es dem Mann.
The man's hat (the man possesses/owns something: Genitive Case - des Mannes) Der Hut des Mannes.
In time you will learn what function a word performs in the sentence, and will know which "case" to use.