"butelkę wody" and "szklankę wody" would be Accusative of "a bottle/glass of water". Like "poproszę szklankę wody" or "Kupuję butelkę wody".
Here, you have quite a different situation. "a glass with water" would be "szklanka z wodą" (Instrumental), but that's also not that. Here you fill the glass with water. You use the water as an Instrument. So it's bare Instrumental without any preposition.
Thank you. I realised about this some minutes later. Anyway, it was useful to learn where the "instrumental" name comes from (using as an instrument. )
I'm guessing 'z' is optional because it is implied by the instrumental case of 'woda'
It's not optional but it shouldn't be there. If you used "z", it would mean that either the water fills the glass together with you or in the glass there is already water and you fill it with something (but not necessary with water).
In English we would rarely say the 'you', it's implied (unless it's a request - would you). Wouldn't "fill the glass with water" be ok?
I concur with the , fill the glass with water, we may say please fill the glass with water but you sounds like a harsh command
But this is just a declarative sentence, not an imperative one. I will change the default version to "You are filling", it makes more sense.
Thanks, there as I've said so many forms of expression in English that don't translate well in other languages. That and the instability of English, every time you turn around, someone changes a word or a sentence, we are in a constant state of flux. Small wonder my have a difficult time understanding us, including the British, the mother language.