"Dewi Lingo wants milk and sugar."
Translation:Mae Dewi Lingo eisiau llaeth a siwgr.
I was marked wrong for 'Mae Dewi Lingo'n eisiau llaeth a siwgr.' It said I used the wrong word and it should have been 'Mae Dewi Lingo'n moyn llaeth a siwgr.' I thought eisiau and moyn were interchangeable?
Yes, mostly but the issue is the grammar changes. The two correct forms are "Mae Dewi Lingo eisiau llaeth a siwgr" and "Mae Dewi Lingo'n moyn llaeth a siwgr". "Moyn" is an actual verb and therefore takes "yn" when used with a form of "Bod" (To be). "Eisiau" is one of the weird nouns which we use in the place of a verb. Since it is not a verb it doesn't take "yn".
is it wrong to say "Lingo yn" in this context? It always seem odd, to me, to contract after names and things but I'm obviously a learner so my intuition doesn't mean anything much.
In the colloquial language it is usual to contract this yn to ’n when it follows a vowel, unless you have a specific reason not to.
Mi ysgifennais i "Mae Dewi lingo yn isio llefrith a siwgr". Beth sy'n anghywir â hynny?