"Man mag Gläser."
So many questions at once, let me try: Man has two translations, 'one' and the indefinite 'you' and I think, they're often interchangable. 'You never know what you get' or 'One never knows' would both be 'Man weiß nie, was man kriegt' in German. Man magen Gläser is no german at all; Magen = stomach. mögen = to like, nominative. Man is somewhat generalising and meant in a plural manner, but uses singular grammar. Duo's translation is fine.