sk gets two different pronunciations.
Before a 'hard vowel' (a, o, u, å) it sounds like SK, like here
Before a 'soft vowel' (e, i, y, ä, ö) you get the SH-like sound that some people might think resembles an F sound.
The hard/soft vowel distinction also influences how K and G are said on their own.
Links to more info about pronunciation (including some awesome videos by Blehg) in the sticky post under Discussions, here: https://www.duolingo.com/comment/5892805
Roughly på corresponds to 'on' and i to 'in'. However prepositions are complex in all languages, so there are lots of cases where you can't translate them that way. But as a general idea it's true.
If the mouse is på skon it is 'on the shoe' i.e. sitting on top of the shoe ('on top of' is ovanpå in Swedish btw) and if it's i skon it is 'inside' the shoe ('inside' is Inuti in Swedish).