Answer:
"You must keep the rules" would be the other (active) way of saying this sentence.
Explanation:
"When a sentence is in the active voice, the subject of the sentence is the one doing the action expressed by the verb. In the passive voice, the subject is the person or thing acted on or affected by the verb's action."
"Rules must be kept" is a passive sentence to begin with. The active version would be "you must keep the rules" or "you must keep rules," depending on the context of the sentence.