"Man mag Gläser."
One does not simply understand this. https://twimg0-a.akamaihd.net/profile_images/2230187991/siM.png
In nominative case: Du, Ihr, Sie and occasionally Man. There are some others in Accusative and dative cases.
It's a so called 'indefinite pronoun' and not neuter. Indefinite, general groups like all, (every)one, none, etc. are in third person masculine singular.
We know translation of "one". It is "Man". What is the translation of others: all, everyone, noone?
All, everyone, everbody means jeder, jedermann. None, nobody means keiner, niemand. Correct me if I got anything wrong, english is just my 2nd language, but I think those words work the same in E. and G.
Is it correct to consider "Man" plural? "Man magen Gläser." What this sentence means then? "Man" has many different translations. How we'll know which is correct and suitable?
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.