Learn Polish: Wypiję pół litra wody, bo jestem spragniony. - I'll drink half a litre of water because I'm thirsty.

The word pół (half) requires the noun that follows it to be in the Genitive case. Since litr is a masculine inanimate noun, its Genitive form is litra.

Does pół also change woda to wody?

Not exactly! The word woda changes to the Genitive form wody because it follows litra. Just like in English we say 'a litre of water', in Polish the 'of' relationship is expressed by putting the second noun into the Genitive case.

Why use wypiję instead of będę pić for the future tense?

Wypiję is a perfective verb, meaning it describes an action that will be fully completed. You are saying you will drink that specific amount of water and finish it. Będę pić means 'I will be drinking', which focuses on the ongoing process rather than the final result.

I see spragniony means thirsty. How would a woman say this?

Since spragniony works just like an adjective describing the speaker, it has to match their gender. A male speaker says jestem spragniony, but a female speaker would change the ending to feminine and say jestem spragniona.