In exercises relevant to the 'Weather' unit, sut  is followed by oedd  (e.g. sut oedd y tiwydd? ), which I thought was a fixed phonetic rule.
Now I'm a bit puzzled...
Can sut oedd  and sut roedd  be used one for the other? The 'Tips and Notes' section of the unit does not mention under what condition roedd  changes into oed .
Thank you for clarifying.

Should I be submitting corrections if it dings me for translating "gwyliau" as "vacation", or is there a separate word for "vacation"?