"Mae e'n mynd i'r pwll nofio."
Translation:He is going to the swimming pool.
I'm generally the last person to point out such things, but I believe that 'pool' is an acceptable translation for pwll nofio. I was marked wrong for not selecting 'swimming pool' but as a native English speaker, i am quite confident that no one would be confused if you said 'pool' instead of 'swimming pool.' After all, how many other types of 'pools' could one go to?
I'm not complaining, but I would encourage duolingo to be a little more flexible on this one :)
The word pwll is used for a pool, pit, pond, puddle, coal mine and in some contexts even for a grave and various other physical and metaphorical ponds and pits.
pwll nofio avoids the potential confusion.