In our first lesson we learned sentences like
Das ist mein Koffer.
Now we're learning about the accusative, and the teacher told us that for both all cases the suffix of the possessive pronoun derives from the ending of the definite article. So, in the accusative it would become meinen Koffer, but also in the nominative meiner Koffer. So, why is it mein Koffer in the example given above? It's nominative because it's predicate, right?
Well, yes. The possessive pronoun for the masculine case is indeed meiner. There are many tables on the Internet where you can look up the correct declension. Here's one of them.
However, note that the examples on that page do not contain a common noun.
In your example above, it's a possessive adjective, not a pronoun. There's not much difference; actually, most people confuse them. The important difference is just that a possessive adjective precedes the noun, the possessive pronoun, however, replaces the noun. This is true for any language (which contains such a thing), including German and English.1
In case of the possessive adjectives, there are some "exceptions" (they're not really exceptions as I will point out below) in respect to the declension. Again, there are many tables, and here's one of them.
Now, why it's not an exception:
When using the plain article you say
Da ist ein Koffer.
and you don't say:
*Da ist einer Koffer.
With that in mind, it's actually not a surprise, that the possessive adjective does not have an ending there.
1 To make the difference between possessive adjectives and pronouns more clear, I take elena's hints to elaborate on this:
In German, you can use the possessive adjective like this:
Das ist mein Koffer.
And if you replace "Koffer" with an possessive pronoun, you say:
Das ist meiner.
In English, there's also a difference between those two kinds of possessive 'indicators' (indicator here is not a proper grammatical term):
This is my suitcase. (-> possessive adjective/determiner)
This is mine. (-> possessive pronoun)
Here's a table on English possessive determiners and pronouns.
Declension of article words and adjectives
As to the declension of adjectives and article words the German system is really complicated. German has three types of declension for adjectives. a) Der Mann ist alt. - The adjective in this position never has an ending and is invariable. b) der alte Mann - In this position the adj. can only have the endings -e/en (I call it simply the n-declension - German grammars like the terms "weak" and "strong" and they use them everywhere, but I think they are more confusing than helpful. c) ein alter Mann - In this position the adj. has the full endings (-er/e/es etc).
There are two sorts of "article words". Those that have the three gender endings -er/e/es such as dieser, jener, welcher etc And there is a second sort, article words such as ein, mein, dein etc which have dropped the endings -er/e/es. As ein/mein etc don't indicate the gender of the following noun any more, these gender endings are added to an adjective when you use an adj. between ein/mein etc and the noun.
So you say "der alte Mann ("der" indicates the gender, so the adj. does not need a gender-ending) and you say "ein alter Mann" ("ein" has no gender ending, so the adj gets a gender ending).
All this is complicated for beginners. And the logic of this system must be understood. And you need a good grammar for beginners with clear declension tables. The only useful grammar for beginners that I know was/is published by Diesterweg Verlag.
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.
The verb "sein" (to be) is an equalizer. Therefore, what follows such a verb is not an object, but rather a copula that takes on the nominative form.
The sentence could just as easily be written "Mein Koffer is das."
But you basically have two "nominatives," one of whom follows the "be" verb.
