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'
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).
why is "ein Apfel" sometimes accepted as an answer in other questions
Because ein Apfel is the correct form in the nominative case, so it's appropriate in sentences such as das ist ein Apfel or ein Apfel steht auf dem Tisch.
Like how in some sentences you need "he" and in some sentences you need "him", so also in German you have ein Apfel in some sentences and einen Apfel in others.
Esst means eating and ist means is.
"means" is not really a good word in this context -- German is not a code for English, where each German word means (replaces) exactly one English word in all of its uses.
Rather, German grammar is different from English grammar, so you may have to add or remove words, or use different translations for one word depending on the situation.
For example, "is" can be translated as ist when it is a copula or linking verb, linking a subject to a predicate (typical an adjective, as in "the cow is big", or a noun, as in "this person is my sister").
But when "is" is used as a helping verb (auxiliary verb), it may have to be translated differently, or not at all. For example, "is" is used in English to form the present continuous tense of verbs, as in "John is eating". German does not use a helping verb for this purpose, so you would not translate "is" separately -- you would translate the whole verb form "is eating" as one piece, into the appropriate German verb form.
Also, as you know if you have been reading the tips and notes (https://www.duolingo.com/skill/de/Basics-1/tips-and-notes , final section "no continuous aspect"), standard German does not distinguish present continuous and present simple in its grammar -- and so "John eats bread" (e.g. every day) and "John is eating bread" (i.e. right now) would be translated the same way: as John isst Brot.
Conversely, you could translate John isst Brot. as either "John eats bread." or as "John is eating bread." You can't say that isst "means" only one or the other -- it can be translated as either, unless there is context which would narrow down the use of a specific verb form in English.
So you have
- ich esse = I eat / I am eating
- du isst = you eat / you are eating (speaking to one person informally)
- er isst = he eats / he is eating
- sie isst = she eats / she is eating
- es isst = it eats / it is eating
- wir essen = we eat / we are eating
- ihr esst = you eat / you are eating (speaking to two or more people informally)
- sie essen = they eat / they are eating
- Sie essen = you eat / you are eating (speaking formally, whether to one person or to several at once)
My answer was counted as being wrong
Then I'm going to bet 50 lingots that it was indeed wrong.
If you have a screenshot showing the exercise and your answer, please show it to us -- upload it to a website somewhere (e.g. imgur) and tell us the URL of the image.
If the mistake is on Duolingo's end, I'll give you 50 lingots.
why einen? What am I not understanding?
The direct object of a (transitive) verb is in the accusative case.
Thus "an apple", the direct object of the verb "eat", has to be in the accusative case in German.
The masculine accusative form of the indefinite article is einen.
Thus Eine Frau isst einen Apfel.