"One one is one."
Translation:Un un yn hafal i un.
I wrote " Un un y un." Which it is. I get that they want me to use the word 'hafal', but is my sentence incorrect?
You're almost right, the most compact way of saying this and the way tables might be recited would be Un un yn un.
The full sentence should be Mae un lluosi un yn hafal i un (One times one is equal to one), but the various shorter forms are much more common.
Thanks for this comment, this sentence had been disabled for various reasons. It's now been updated and edited.