Oh, a possible response you might get if you ask someone if what they're drinking is vodka ^_^
Isn't it strange to say no, after saying an affirmative sentence? It didn't make sense to me!
Rather before. Anyway, it's like: "Are you drinking beer? No, I am drinking water".
The answer here should be "No, I am drinking water." In idiomatic English, "I drink water" does not mean the same thing. "I drink water" implies that you do it habitually, and I'm pretty sure we'll find there's a different verb form for that in Polish.
Yes and no.
"No", because: in 99% of cases (and this 1% is almost only verbs of motion), in Polish you don't see a difference between Present Simple and Present Continous, and they are translated exactly the same way.
"Yes", because: actually there are several habitual verbs, and there is a habitual equivalent of "pić": "pijać". So you could say "Nie, pijam wodę". Although here it seems a bit strange to me, if you added "tylko" (only), that would make more sense.
"No", because: these are not in such a common use, and they are not taught in this course (accepted when we remembered to accept them) - as even for the verbs that have a habitual form, still, the normal one is perfectly fine and correct.
What if you want to say "No, I'll drink water." Because I typed that thinking "No, I drink water." is a weird sentence on its own. But got marked wrong.
Well, this is the beginning of the course, there is very little vocabulary and grammar introduced so far - many sentences will not feel that natural. This looks like an answer to a question like "Do you drink beer?" - although I guess it would feel better in Present Continuous. I changed it so that will be the default version.
As for "I will drink", that would be "wypiję", but it implies drinking (totally) something specific, like a glass of water. "Będę pić wodę" (I will be drinking water) feels more natural.
Imperative? Imperatives have their own forms, of course.
"Nie pij wody!" (2nd person singular)
"Nie pijcie wody!" (2nd person plural)
This answer implies that a question was asked beforehand. It is a response to a question; otherwise, the "No" is superfluous. Without a question beforehand, only a declarative sentence is appropriate: "I am drinking water."
In speech? Well, some people nasalize the final e more clearly, but it is quite likely that only context will make it possible to distinguish them.
Shouldn't it be "Nie, jestem piję wodę." instead of just "Nie, piję wodę."? I'm pretty sure "Nie, jestem piję wodę." just means "No, I am drinking water." as to "Nie, piję wodę." just meaning "No, drink water."
No, "Jestem piję wodę" makes about as much sense as "I am I drink water" - absolutely none.
Don't treat the Present Continuous construction "I am x-ing" as having two verbs, it only has one. Don't translate "to be".