"The ruler is broken."
Translation:Mae'r pren mesur wedi torri.
OK, I've had rethink about this, your suggestion is correct because this is a passive sentence, the breaking has happened to the ruler, rather than the ruler doing the breaking.
The full sentence is:- 'Mae'r pren mesur wedi cael ei dorri' = 'The ruler has been broken' (lit:- The ruler has had its breaking)
This is very often abbreviated as in your suggestion:- 'Mae'r pren mesur wedi'i dorri'
Finally this is contracted even more to:- 'Mae'r pren mesur wedi torri' - 'The ruler is broken'
However this is not the same as 'Mae'r pren mesur wedi torri'r cwpan' = 'The ruler has broken the cup'
So, your suggestion has been added now. Thanks for the comments.