"I go swimming twice a month."

Translation: Dw i'n mynd i nofio ddwywaith y mis.

[MatthewSke1]

Why is dwywaith mutated?

[ibisc]

Because it is being used as an adverb - just another of the many things causing a soft mutation.

[Brynle]

Why is it "mynd i"..and not just mynd followed by a verb?

[ibisc]

Many welsh verbs/verb-nouns are followed by prepositions to convey particular meanings. Some examples with mynd:

• mynd - going, to go
• mynd i rywle - to go (to) somewhere
• mynd at rywun - to go to someone
• mynd i wneud rhywbeth - going to do something
• mynd â rhywbeth/rhywun (i rywle) - taking something/someone (to somewhere)
• mynd o - to go from
• mynd i nofio - going/to go swimming