Learn French: Je n'ai pas encore passé l'aspirateur dans le hall.

In French, short negative adverbs like pas encore wrap around the conjugated verb. In the passé composé, the conjugated verb is the helping verb (ai). Therefore, pas encore goes directly between the helping verb and the past participle (passé).

Why do we use the verb passé (passed) here to mean "vacuumed"?

French doesn't have a single everyday verb that means "to vacuum." Instead, you use the expression passer l'aspirateur, which literally translates to "to pass the vacuum cleaner."

Why does the French sentence use dans le hall (in the lobby) when the English translation is just "vacuumed the lobby"?

In the French expression, the vacuum cleaner (l'aspirateur) is already the direct object of the verb. Since you are literally "passing the vacuum in the lobby," you need the preposition dans (in) to explain where you are doing it.