"Yes, they may"
I'm not sure why "Cân, cân nhw" is wrong. A lot of the Welsh sentences seem to express "yes" by repeating the first verb.
Thank you, as ever, for the response and the link. At the moment, as your notes suggest, it is rather too full and advanced for me, But it seems very clear and logical. What puzzles me, though, is how to find this teaching on the DL site without this link - where is it?
The 'tips and notes' are set out at the beginning of each section as you start it, and you can get to them through a link at the top left of the practice window as well.
They are not available in the app - you need to be on the web-site.
Those particular notes on yes/no are not embedded in the system, though - they were put together in response to a number of queries about yes/no in a number of sources of feedback.
However, if you want more than a series of short notes on Welsh language patterns, and your interest is likely to go beyond what is in Duo, you could look out for a copy of a good modern grammar book such as 'Welsh Rules' by Heini Gruffudd.