"Megan, do you like lemon?"
Translation:Megan, dych chi'n hoffi lemon?
See the notes for 'Present Tense 1/2'.
yn/'n is required to link forms of bod (being/to be) (dw i, dych chi, etc) with verb-nouns such as hoffi (to like, liking) and, as you will see later, with adjectives and nouns.
The pattern that you met with eisiau (a want/need) is exceptional in Welsh - no yn/'n is needed - as it has developed from earlier and more formal patterns through frequent usage.