Questions & Answers about Wir packen heute einen Rucksack.
Why is it einen Rucksack and not just ein Rucksack?
Because Rucksack is a masculine noun and it is the direct object of the sentence (the thing being packed). This means it is in the accusative case. In the accusative case, the masculine indefinite article ein changes to einen.
How would I change this to say we are NOT packing a backpack?
You would use the negative article keinen instead of einen. The sentence becomes Wir packen heute keinen Rucksack. Just like einen, keinen gets the -en ending because the masculine noun is in the accusative case.
Why is heute in the middle of the sentence instead of at the end?
In German, time words like heute usually come very early in the sentence, right after the verb. German prefers to state when something is happening before stating the direct object like einen Rucksack.
Why doesn't the German sentence have a word for are in are packing?
German does not have a separate continuous tense like English does with are packing. The single verb packen covers both we pack and we are packing.