One must keep one's promises.

One's promises are kept

Ones promises must kept

Ones promises were kept

One's promises must be kept

All of the above

Keep is a verb which means to maintain.

Kept is the second and third form of Keep.

The subject 'One' has to be replaced with 'One's promises' keeping the tense same.

Active voice: One must keep one's promises.

Passive voice: One's promises must be kept