"She ate rice yesterday."
Translation:Bwytodd hi reis ddoe.
In what contexts would it be preferable to use these different forms? Or are they interchangeable?
The short form of the verb (bwytodd hi) and the form using the simple past of gwenud as an auxiliary (gwnaeth hi fwyta) are equivalent. Which one to use is often a matter of personal style, although the short-from verb is perhaps more formal. One version or the other might be used more often in a particular dialect, but you will come across both forms in use in all parts of Wales, really, with various dialect variations.