So, what am I drinking then? … worried look into glass
Well, if I were you, I'd prefer water over bad beer ;)
The word order is confusing me.
Here "pivo" is new information, so it goes at the end.
In the previous question "vodu nepijou", was "vodu" a known topic and that's why it began the sentence?
Yes, often it is like that.
I'm sorry if this gets explained in a later lesson but I couldn't find it up to this point. Is there a grammatical difference in Czech between You do not drink and You are not drinking?
Not in Czech. Both are "Vy nepijete".
What would it be like if it was "Don't drink beer!" (imperative form)?
There are two options: "Nepij pivo!" (informal singular); "Nepijte pivo!" (formal singular and plural)
So would "Pivo nepijete" be used to say "You don't drink beer?"
Depends on the questionmark (or the intonation), really. "Pivo nepijete." can also be just "You do not drink beer.".
Could "Nepijete pivo" be spoken as a command to somebody, or is this just a statrment of fact?
This is a statement. Czech imperatives are different and will be shown later. (Nepijte pivo!)
Why would piva be incorrect? Isn't beer in the accusative case in this sentence?
Yes, it is in the accusative case. But pivo is a neuter noun, and the accusative singular is the same as the nominative singular. Piva would be nominative and accusative plural.