Learn Russian: Можно арендовать зал на три часа?

We use на plus the Accusative case to express intended duration. When you rent, borrow, or go somewhere for a certain period, the time you plan to have it is the intended duration. If you were just stating how long an action actually took (like reading for three hours), you would just say три часа without the на.

If на requires the Accusative case, why does the word часа look like it is in the Genitive case?

This is because of how numbers work in Russian! The preposition на puts the number три into the Accusative case (which for inanimate concepts looks exactly like the Nominative). However, the rule for the numbers 2, 3, and 4 is that they force the noun immediately after them into the Genitive singular. So, часа is Genitive singular because of the number three.

Why is there no subject like 'I' or 'we' in this sentence?

The word можно is an impersonal word meaning 'it is possible' or 'one can'. By not adding a specific person (like мне можно for 'I can'), the sentence becomes a very natural, polite, and general inquiry: 'Is it possible to rent...' or 'Can one rent...'.

Is the word зал (hall) in the Nominative or Accusative case here?

It is in the Accusative case because it is the direct object of the verb арендовать (to rent). It looks exactly like the dictionary form (Nominative) simply because зал is a masculine, inanimate noun, and masculine inanimate nouns do not change their endings in the Accusative case.