"I go swimming twice a month."
Translation:Dw i'n mynd i nofio ddwywaith y mis.
Because it is being used as an adverb - just another of the many things causing a soft mutation.
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