Do the "ti" and the "yn" have to be written (and said) separately, or can you say "Wyt ti'n dost"?
You are right, it should usually be wyt ti'n dost - the sentence will be corrected shortly, although it may take a while for the change to go live.
As previously mentioned this is usually 'wyt ti'n dost'. We had some problems creating the course and didn't manage to include 'wyt ti'n' until after this unit and because of the strange rules of the Duolingo system we couldn't include it here.
However this is still a correct sentence in the context of emphasis. This sentence could be transposed as 'Are you (really) ill?'
Now fixed in version 2. Thank you.
The deletion of the old sentence and the addition of the new one may take a little while to filter through the system.
mewn poen would be more usual for 'in pain':
- Dych chi mewn poen? - Are you in pain?