The sentence "One should keep their promise" is difficult to convert into a true passive voice construction while preserving its meaning. Passive voice focuses on the action being received rather than the actor performing it.

Here's why it's tricky and the closest possible phrasing:

• The Problem: The original sentence is a general statement about obligation. The active voice emphasizes the individual fulfilling the promise.
• Attempted Passive (and why it's awkward): A direct passive construction would be: "A promise should be kept."
  • While grammatically passive, this loses the original sentence's emphasis on who should be keeping the promise. It becomes a general statement about promises, not about the obligation of the person who made it.

Therefore, while a technically correct passive voice construction exists, it loses the original meaning.