Prepositions are always difficult, because different languages use different prepositions. I think 'om' is the easiest. It almost always corresponds to 'if' in English. 'På' literally means 'on' and 'till' means 'to'. In some situations we use these prepositions differently than English. I think the best way of acquiring a feeling for when to use which preposition is to read a lot of Swedish :) Edit: I was too fast regarding 'om'. We use it in other cases as well. "Två gånger om dagen" = "twice a day". There is no logic, just one of the things different languages handles differently.

Dziękuję bardzo, Konrad! Te przyimki doprowadzą mnie do szaleństwa! :-)

Cześć! There are all main descriptions about "om" "till" and "på" with simple examples and notes: https://en.wiktionary.org/wiki/p%C3%A5#Swedish https://en.wiktionary.org/wiki/om#Swedish https://en.wiktionary.org/wiki/till#Swedish Take in consideration that swedish "i" is also not strictly equal to english "in" :)