"One must apply the rules."

Translation: Oni devas apliki la regulojn.

[EricVandenburg]

isn't "oni apliku la regulojn" the same thing?

[mizinamo]

No, not quite the same thing. That's more like "One should apply the rules."

"Kion oni faru nun? - O, oni apliku la regulojn." "What should we/one do now? - Oh, one should apply the rules."