"I work on the checkout."
Translation:Dw i'n gweithio ar y ddesg talu.
Would Dw i'n gweithio WRTH y ddesg talu. "I work AT the checkout" also be possible?
ar sounds like "on top of (the checkout desk)" to me.
Prepositions are hard :)
If you are studying Welsh in the long term, beyond this introductory course, you could try and find a used copy of 'Y Geiriau Bach' (by Cennard Davies) which gives detailed examples of the many uses of Welsh prepositions.
Sadly, it is now out of print - perhaps if enough people contact the publishers they might at last take the hint and re-issue it!