"Do you eat corn?"
Translation:Manges-tu des maïs ?
'mais' in French and 'corn' in English are mass nouns (like 'sugar', i.e. they don't have a plural form). Therefore, the best answer would be 'mange-tu du mais'. 'mange-tu le mais' is also ok, but mange-tu des mais is simply ungrammatical. this is a bug and needs to be fixed.
how "du mais" and "des mais" are both correct - is "mais" singular and plural?
I assume it's similar to corn in english. You could eat "a corn" - a baby corn, or a corn on the cob, where it makes some sense to think of it singularly - or you could eat "some corn" - loose kernels, perhaps, where you'd think of it more by a portion rather than individually.
For the multiple choice question I only picked "Manges-tu des maïs?", but "mangez-vous le maïs?" is also listed as a translation of "do you eat corn?" which I am a bit confused about ... Perhaps the different meanings are more apparent in French?