"Do you like the crab?"
Translation:Dych chi'n hoffi'r cranc?
I think you use yn all the time unless the verb that follows is irregular. You never use yn with isio/eisiau. I think you just have to learn which ones don't use yn.
You don't use yn with "eisiau", because it is noun and not a verbnoun, but it acts like one; there are five irregual verbs, bod = to be; gwneud = to do/make; gwybod = to know; cael = to have; dod = to come; mynd=to go; the last four only have irregual pretite forms