German inflects its adjectives, articles and pronouns in three cases: nominative (for subject and subject complement), accusative (for direct object and with some prepositions), dative (for some other prepositions).
The accusative of indeterminative article changes just for the masculine gender: EIN/DER are the masculine articles for subjects or for names which came after to be (or other copulas);
EINEN/DEN are the masculine articles for direct odjects (that come after transitive verbs).
they are grammatical cases ( https://en.wikipedia.org/wiki/Grammatical_case ) in some european languages they are used to express the role of a word in a sentence.
Even English have some words that are declinated in three "cases" (nominative and accusative) they are I, you, he, she, it, we, they.
You can imagine that I is in nominative case, because it has the function of subject, me is the accusative case because it's used for the direct object. (it is not grammarly correct, but it can help you to understand)
cases are this: something that change words without changing their meaning, but changing the meaning of the sentence: I am me! "I" and "me" is the same person! but "me like choccolate" doesn't mean "I like choccolate" because they have different functions.
I hope it helps you: I don't know how to explain it more simply :)
It depends on the case or the sentence - Nominative, Accusative or Dative. Nominative: Masculine = Ein, Feminine = Eine, Neutral = Ein (Plural = Eine) Accusative: Masculine = Einen, Feminine = Eine, Neutral = Ein (Plural = Eine) (ONLY MASCULINE CHANGES) Dative: Masculine = Einem, Feminine = Einer, Neutral = Einem (Plural = Einen) (THEY ALL CHANGE)
The ending applies to Der, Die and Das / Mein and Kein also.... for example
Nominative: Masculine = Der/Ein/Mein/Kein. Feminine = Die/Eine/Meine/Keine. Neutral = Das/Ein/Mein/Kein Accusative: Masculine = Den/Einen/Meinen/Keinen. Feminine = Die/Eine/Meine/Keine (same as nominative). Neutral = Das/Ein/Mein/Kein (also the name as nominative) Dative: Masculine = Dem/Einem/Meinem/Keinem. Feminine = Der/Einer/Meiner/Keiner. Neutral = Dem/Einem/Meinem/Keinem
I really hope this helps! To explain when a sentence is Nominative, Accusative or Dative is much more complicated!
For example: die Frage (the question) - eine Frage, accusative: Ich habe eine Frage. Das Brot (bread) - ein Brot, accusative: Ich habe ein Brot. Der Apfel - ein Apfel, accusative: Ich habe einen Apfel. Only the masculine articles der and ein change in the accusative case to den and einen. The neuter ein (Brot) does not change in the accusative.
sie is called they and sie is also she ....how?
Just how it is in modern German. Probably through sound changes, where final vowels tended to get unstressed.
The words used to be distinct (siu, sie) a thousand years ago or so but have been identical for centuries now.