"It was windy this morning."
Translation:Roedd hi'n wyntog y bore 'ma.
Why do you think so? It's a perfectly normal form and I'm not aware of any exceptions that would apply here.
Are you sure you're not thinking of "Mi oedd..." which is a pretty common form that I use myself. The issue with "Mi roedd..." is that the "r" at the start is already the remenant of another verbal particle "yr".
I... don't think so? I can see what you're getting at, but if that's true it's the first time anyone's told me so in about ten years (I spoke decent Welsh once but moved away and am brushing up again). And there seem to be plenty of examples when I searched, including literature.
I chcked for "mi oedd" as well and it's more common in search results, but actually most of the examples turned out to be the pronoun, for example, "Do'n i 'rioed o'r blaen wedi cyfarfod pobol yr un oed â mi oedd yn gallu dyfynnu enwogion mewn sgwrs bob dydd, a dyma ddwy o fewn munudau i'w gilydd". But I did find some where it's the particle.
Annoyingly I don't have any reference books in this flat, because I'm not sure what to think now.