does 'do, roeddet ti yna' not make sense, or is it just incorrect for this lesson?
do/naddo are used as yes/no with the simple past tense.
With the imperfect tense (roedd- forms) you need to use the corresponding oedd-/ nac oedd-:
Be interested to know why the treiglad 'f'uest ti.
A mistake, sorry. There is no reason to mutate here and the answers now read:
Is Buest, mi fuest ti yna. possible?
Or is only do/naddo possible for this tense?
(Also, are mi fuest ti etc. accepted alongside buest ti?)
do/naddo here - it is a simple past tense.
Some dialects do sometimes put mi in front of present and imperfect verb-forms of bod, but I don't know whether it is done for other tenses. If in doubt, leave it out - it is not required.