"Trowch i'r dde nawr."
Translation:Turn right now.
de takes a soft mutation because of the definite article -- in the meaning "right", de is feminine, and feminine nouns take soft mutation after the definite article.
i'r de would be "to the south" (de is masculine in that sense) but i'r dde takes mutation just as i'r dafarn "to the pub" would for those speakers who treat tafarn as feminine.
I see! Thanks a lot for the clarification. I had wrongly thought that the mutation depended on the use of i.
By the way, I was also wondering whetherde (south) is distinguishable from de (right), besides the context of speech, and your answer covered this issue, as well.