Learn German: Aber der Koffer ist alt. - But the suitcase is old.

In German, every noun has a grammatical gender. The word Koffer (suitcase) is masculine, so it takes the masculine word for 'the', which is der when it is the subject of the sentence (the nominative case).

Why doesn't the word alt change to match the masculine der Koffer?

When an adjective comes after the verb ist (is) to describe the subject, it does not get any special endings. It just stays in its basic dictionary form: alt. You only add an ending if it comes directly before the noun, like in der alte Koffer (the old suitcase).

How would I negate this sentence using kein?

You actually cannot use kein here! Because your current lesson is about kein, it is important to remember that kein is only used to negate nouns (like saying 'That is not a suitcase'). To negate an adjective like alt, you must use nicht. So you would say Der Koffer ist nicht alt (The suitcase is not old).

Does the word aber change the word order?

No, aber (but) is a friendly conjunction that does not mess with the word order. It sits at the very front in 'position zero'. The actual first block of the sentence is der Koffer, and the verb ist stays exactly where it should be: in the normal second position.