all words show the potential or possiblity
that is janu " to know "( potential is there that is you might know the thing ) and similar word janunga " i will know ".( potential is there that in future you will know about that thing )
similarly karu (karunga) ( likey to do ) { possiblity or potential that you will be doing that )
and mangu ( manguga) ( i will ask for it ) ( again it show possiblity that you will ask for something )
janu , karu, mangu you can think.them as the short form of these words manguga , karunga,janunga ( these are bit polite to say also )