"Dw i'n mynd nofio ddwywaith y mis."

Translation: I go swimming twice a month.

[LangForThought]

Why is it "ddwywaith" here instead of "dwywaith"?

[ibisc]

dwywaith y mis is used as an adverb here, and so the initial letter of the phrase gets mutated.

(Duo also has a typo in this sentence - it should read ...mynd i nofio...)