"Do you like milk?"
Translation:Dych chi'n hoffi llaeth?
Wyt ti'n hoffi llaeth? is an accepted translation.
The 'n is required to link the verb (wyt ti) to the verb-noun (hoffi) - see the course notes.
To see how to find the course notes generally, go to https://forum.duolingo.com/topic/924/hot and read the discussion 'Course tips and notes'. The 'duome' link there is useful for browsing all the notes in one place. We recommend reading the notes for each new section as you start it.