"Does he want a trifle?"
Translation:Ydy e eisiau treiffl?
'Moyn' is an abbreviated form of the verb-noun 'mofyn' or 'ymofyn' (requiring, seeking). Some dictionaries may not have 'moyn' but they should have one or both of 'ymofyn' or 'mofyn'. Since it is a verb-noun it should be used with yn/wedi (even though the course lessons do not seem to show it - reported already):
Mae e'n moyn treiffl - he wants some trifle or he wants a trifle or he wants trifle.
Ydy e'n moyn treiffl? - does he want some/a/- trifle?
Dydy e erioed wedi moyn treiffl o'r blaen - he has never wanted trifle before.
'Eisiau' and 'angen' are not used with yn/wedi in the same way because they are, erm.., let us say... nouns being used in a non-standard way in the particular expressions 'dw i eisiau..', 'dw i angen...', etc.