If you saw a sign next to a river saying "Don't drink the water," would it even have उस in it? Or would it be enough to just say "पानी मत पियो"?
Because if you don't need any indication of "the" in a normal context, then "Don't drink the water" should be accepted as a translation for this. (Posting this comment because I also tried it and was rejected.)
These sentences are context-free, but we need to imagine a context to create meaning. It appears that English speakers assumed that "Don't drink water." was just too far fetched to be right, because the context for saying that is almost unimaginable. Is the Hindi sentence just as unlikely as the English one?
I have gathered from these lessons that in some cases a Hindi sentence can be translated into English with either "the" or "a" and that the definiteness/indefiniteness is contextual rather than explicit. What is it about "पानी मत पियो" that has to be interpreted generically rather than specifically? Could there be a context in which it means "Don't drink the water"?
Since the to drink is पीना, I was expecting this to be पीयो. Why is it पियो?
A few verbs in Hindi are a little unique like that, long ई gets changed to a short इ for certain conjugations. Some other examples of odd verb forms: Past tense of देना = दिया/दिये/दी, imperative form (तुम) of देना = दो, imperative form (आप) of देना = दीजिए, past tense of लेना = लिया/लिये/ली, imperative form (तुम) of लेना = लो, imperative form (आप) of लेना = लीजिए.