"I have been working on it for five months."
Translation:Pracuji na tom pět měsíců.
So na tom is in the locative and pět měsíců is in the genitive? Anyone know a good guide on which case to use in different situations?
I think that měsíců is in the genitive because that's how you use numbers 5 and above (Tips and Notes https://www.duolingo.com/skill/cs/Numbers-2 and this also talks about the cases used with some prepositions) but pět and the whole noun phrase is either nominative or accusative ( https://en.wiktionary.org/wiki/p%C4%9Bt ) but I'm not sure which. The wiktionary articles for prepositions 'na' ( https://en.wiktionary.org/wiki/na#Czech ) and 'v' ( https://en.wiktionary.org/wiki/v#Czech ) talk about when they take which case.