"You are drinking water."

Translation: Pijesz wodę.

[jess.a.hay]

Why is it 'pijesz wode' and not 'pijesz woda'?

[Jellei]

Well, that's the 'fault' of Polish case system. I see you're rather at the beginning of your adventure with Polish, you will soon come around many comments that ask similar questions and Polish people answering them in details :)

In this particular example: Pijesz kogo? co? = you drink whom? what? (forget that it doesn't make any sense to 'drink someone' --> Accusative. Thus, "wodę'.

[BenConway6]

A funny thing about English is this sentence can also mean 'jesteś wodą do picia'.

[Jellei]

True :D But maybe let's not make this an accepted answer...