This sentence doesn't seem to be grammatical, even in the English translation. Markers of time and the perfect tense don't normally jive, right?
I could see, "Ddoe, nes i gael bath/ Ddoes, ges i fath." (Yesterday, I had a bath).
The course makers seem to want to avoid dealing with mutations at an early stage, which is fair enough, but this does seem a little unnatural. This could be avoided if they used "heddiw" instead of "ddoe". Worth suggesting?
I think so. Wanting to avoid mutations makes sense, and you see it other places, too. But this just doesn't seem right to me. Do any native speakers want to weigh in? Nic, are you?
I'm a second-language speaker, and have been teaching Welsh to adults for 18 years. Things like "Dw i wedi cael bath ddoe" are among the incorrect examples we use to explain the difference between the perfect and simple past tenses.