English elementary I
In English language there many prepositions. They are mainly used to describe location, position, time constraint.
In - means position of an object being inside, mean time (after). Examples: in a house, in a box, in an hour, in 2 weeks, in 15 years
On - means position of an object being on top of something, means time with week days: On a table, on a blackboard, on top of a hill, On Monday, on Teusday