I can see that "yn caerdydd" isn't what they wanted, but is it actually wrong?
Thanks, ibisc, but what I meant was that all I had to go on was just the 2 words "In Cardiff". I don't see anything there that will tell me which version will be needed. Normally I would take it from the context but there was no context in that particular question. I think I should have been allowed to get away with it.
'In Cardiff' will always be Yng Nghaerdydd - there is no other way of putting it.
I have just added the whole phrase 'yng Nghaerdydd' to the pop-up hints, and I will hunt out which 'Hints and Tips' have the full explanation.
Edit - I have just added an explanation of yn (meaning 'in'), the nasal mutation and the related changes to yn (-> ym, yng), to the notes for 'Countries'.
Thanks very much, ibisc. You're a trooper. To answer my original question then, "in Cardiff" will never be Yn Caerdydd, but "Yng Nghaerdydd" every time. Am I right? Thank you for being there to help when we get stuck: You're great.