Accusative and genitive have same form for masculine animate gender. But you can recognize concrete cases from the sentences.
We learn at school "case questions"and a "help word" for every case.: For genitive is it (BEZ) koho? čeho? (without) who what? and for accusative - (VIDÍM) koho? co? ( I can see) who what?
for example: GEN: (bez) psa, but (bez) stroje and ACC: (vidím) psa but (vidím) stroj.
Thanks for your answer. I understand your use of the cases as it is similar to German (so far). I only wondered because this very sentence in Polish would use an accusative object in a positive statement (I see that dog) and Genitive in a negative statement (I don't see that dog) Of course in the sg it would make no difference in czech but it would in the plural. So I suppose if you use Genitive both ways, it should be:
ty psy nevidím - I don't see those dogs.
Seems Czech has some tracks of it in proverbs and archaic language, right? E.g.: "Nechval dne před večerem." (Don't praise the day before the evening/ roughly means: Don't get your hopes up too early)
Also constructions with ani (ni) apparently, though it's very bookish and archaic language which is beyond our level still. :p