Breakdown of Packst du keinen Koffer?
du
you (informal singular)
kein
no, not any
der Koffer
the suitcase
packen
to pack
Questions & Answers about Packst du keinen Koffer?
Why is it keinen instead of kein?
Koffer is a masculine noun (der Koffer). Because it is the direct object (the thing being packed), it takes the accusative case. In the accusative case, masculine articles get an -en ending, so kein becomes keinen.
Why is the sentence translated as "Aren't you packing..." instead of using nicht?
In English, we usually negate the verb ("Aren't you"). But in German, when you negate a noun that would normally have an indefinite article (like "a suitcase"), you must use a form of kein. Literally, this sentence is asking "Are you packing no suitcase?"
Why does the verb Packst come at the very beginning of the sentence?
This is a yes/no question. Just like in English where we often start with "Do" or "Are", in German you form a yes/no question by simply moving the conjugated verb to the very first position.
Why does the verb end in -st?
The subject of the sentence is du (the informal "you"). The base verb is packen (to pack), and to make it match du, you remove the -en and add the -st ending.