"Your dog's nose is very long."
Translation:Mae trwyn eich ci'n hir iawn.
I believe that 'dy' is followed by a soft mutation, but 'eich' isn't. Welsh possessive pronouns don't all follow the same pattern.
The grammar notes about this were very useful. If you can't find them and would like them, I'd be happy to copy them out and post them on here. :)
No need to post the notes here; they are readily available... At the top of this page, click on 'Topic: Welsh', then on 'Popular', then look for the sticky discussion 'Course hints and tips'.