It would be nice if everything were regular and we had "meiner" and "meines", "deiner", "deines" etc. in the nominative (as happens with determinate articles and other so-called "D-articles"). That's however not the case.

In the nominative, masculine and neuter possessive articles have no ending (or "signal", as it's sometimes called). In the accusative, nominative gets an "en" ending, while neuter continues with no ending. With dative and genitive, all genders finally get proper endings.

In your example, "mein Koffer" is in the nominative because with some verbs (the most important of which is "sein") there can be no direct object, so what comes after them stays in the nominative.

It would be nice if everything were regular and we had "meiner" and "meines", "deiner", "deines" etc. in the nominative (as happens with determinate articles and other so-called "D-articles"). That's however not the case.

In the nominative, masculine and neuter possessive articles have no ending (or "signal", as it's sometimes called). In the accusative, masculine gets an "en" ending, while neuter continues with no ending. With dative and genitive, all genders finally get proper endings.

In your example, "mein Koffer" is in the nominative because with some verbs (the most important of which is "sein") there can be no direct object, so what comes after them stays in the nominative.