A noun is a word that means “a thing” like “Apple” and “Peter”.
In English where a noun appears in a sentence can help you understand if the noun is *doing something* or *having something done to it*.
For example “Peter ate the apple” vs “The apple ate Peter” are clearly very different sentences even though the words “Peter” and “Apple” are the same, their *place* in the sentence controls their meaning.
Sometimes in English we will change the spelling or “case” of the noun we use depending how it’s used in a sentence for example.
“He gave it to me” vs. “I gave it to him”. In this example both he and him, and me and I refer to the same people, but we use different words because of how they function in the sentence. But English still relies on word order very strongly so “Him gave it to I” wouldn’t be easily understandable to an English speaker. We wouldn’t know whether to use the word order, or the form of the word to control who is acting and who being acted upon.
In these examples “He” is the *nominitive case* of a noun and “him” is the *accusative case*.
Nominative is the noun doing an action, accusative is the noun that’s being acted upon.
*Nominative* ate the *accusative.*
*Nominative* jumped over the *accusative.*
*Nominative* clearly needs to understand *accusative* better before trying to learn a new noun.
In English other than pronouns we don’t (generally) modify a noun if it’s nominative or accusative, we rely on word order, but in German they do modify the noun per your eine/ein/einen example.
In fact in German a sentence “Him gave it to I” would be understandable because they are so much more fixated on the case than the word order.
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