Distinguish duration, a future point and elapsed time rather than translating a preposition word for word.
I for duration in common patterns
I commonly expresses a duration such as two hours or three years. Choose the verb tense according to whether the period is completed or continuing. Compare full sentences rather than isolated prepositions.
Jeg har bodd her i tre år.
I have lived here for three years and still do in this context.
Future om and past for … siden
Om with a time amount can mean that far into the future, while for … siden points that far into the past. Changing the expression changes the time even if the number stays the same.
Vi møtes om to dager. Vi møttes for to dager siden.
We meet in two days; we met two days ago.
På can describe time needed to finish
With a completed task, på can describe the time needed to finish it. This is not its only time use; it also appears with days and occasions. Learn full expressions instead of assigning one meaning everywhere.
Jeg leste boka på to dager.
I finished reading the book in two days.
Try it yourself
Write about how long you have studied, a future appointment and a past event. Compare i, om and for … siden.