The best versions to stick with are arnon ni and arnyn nhw - these are the colloquial usages taught in Welsh for Adults and in schools, and will avoid confusion with any formal or dialect forms. If Duo offers up anything else, just raise a query through the report function.
There are other, formal forms in some dictionaries etc (arnom and arnynt in this case), but you are unlikely to meet them in this course or in the wild until you start reading more formal texts.
Yep, the prepositions are affected quite a bit by dialect and traditional vs modern.
It would be good to have a heads-up for arnyn in the tips and notes. They currently just mention arnon.
The tips and notes for 'Health' already have a table showing the full conjugation of ar.
Thanks ibisc. When I wrote the comment, I'm sure I double and triple checked the table, which I'm sure had arnon for both ni and nhw, but having just gone back to it now, it does have arnyn for nhw, so either it's been changed or I'm going mad, or both! Thanks for your help.
Dim prob. After I had checked it was there I did go over it again and found the typos!
The usual forms are definitely arnon ni and arnyn nhw.
Great, thanks! By the way, I just reported a strange character in one of the picture choice questions, but then realised it was actually the number 3 overlayed on top of the word. So it's just a layout thing and not something I think you have control over. Sorry!