Why is mleka genitive here? I would say it is accusative (or dative?). And can you say 'butelka z mlekiem'?
Yes you can say "butelka z mlekiem" , but then it is "a bottle with milk. "
Genitive case is the one that is listed second on the declension charts, ( here is how to use Wiktionary and WSJP to get conjugation, you can access declension the same way )
Genitive is used:
to indicate possession, a noun in Genitive goes second; Kasia's dog - Pies Kasi
after negation of the verbs, that need accusative (as a direct object)
as a direct of some verbs, some of them are seen as negative, like nienawidzić- hate; szukać- search, negować-negate, brakować- lack, odmawiać- refuse, but also some other ( I will link a list)
after some numerals and quantifiers ( I guess that is for a lesson on numerals)
after some prepositions ( z, ze in the from meaning) https://en.wikibooks.org/wiki/Polish/Prepositions_as_hints_to_declensions
link to a list of verbs https://www.duolingo.com/comment/12409861
Also awesome comment by br0d4 here https://www.duolingo.com/comment/13769639
Cześć! So, is the milk usually sold in bottles in Poland? W Finlandii nie mają butelki mleka. :) How would you say carton of milk? Also how popular are the options for milk, like oat drink or soydrink, in Poland? :) Dziękuję!
Karton mleka :)
I'd say that mostly it's sold in cartons, but if I saw a bottle of milk I wouldn't pay attention to it, it wouldn't feel weird.
I don't think those options are popular, but maybe it's just my lack of knowledge.