"I want the chocolate and the coffee."
Translation:Dw i eisiau'r siocled a'r coffi.
How was I supposed to know "Rydw i moyn y siocled a'r coffi" was a correct translation as well?
Well the course doesn't really teach "moyn or "Rydw i" since they are not as common as the other variants i.e "eisiau/isio" and "dwi/ dw i". So there would be no way for you to know unless you'd come across them before as answers.
Now you do :)
I'm guessing this was a multiple-choice question -- they seem to like using them to "broaden our horizon" and introduce variants of grammar or vocabulary, such as "dy/rydw" or "eisiau/idio/moyn".
So you're just supposed to learn them from seeing them in such contexts, getting them wrong and then subsequently remembering them.
From what I've heard, though, the sentence you quoted is actually wrong -- eisiau doesn't take yn but moyn apparently does.