"Mae hi'n un ar ddeg o'r gloch."
Translation:It is eleven o'clock.
November 21, 2016
It depends on the context. Generally it is a preposition meaning 'on', but that is not to say that it is always best translated like that:
- Mae'r gath yn eistedd ar y bwrdd - The cat is sitting on the table.
- Un ar ddeg (= Un deg un) - Eleven ( the traditional un ar ddeg version is generally still used with time).
There are other uses of ar as well which you may come across from time to time.