"On Friday I'm going to Aberdaron."
Translation:Ddydd Gwener dw i'n mynd i Aberdaron.
Why is one mutated and one not? The mutated one doesn't have anything before it, so why is it mutated? could you use both versions as a sentence starter?
Nowadays, Ddydd Gwener... is used to start a sentence where the English meaning is 'On Friday...'. Not so very long ago, the unmutated version would often have been used at the start of a sentence, and the mutated version ...ddydd Gwener... only after the main verb.
In this case, the mutation is not caused by anything come before the word, it is mutated because it is being used as an adverb of time - a word or phrase saying when something happens.
You can use the unmutated word to start the sentence, but the mutated version is more common now and we are gradually bringing this course into line with that practice.