One basic way to think about the 'pure' forms is to add "top of" for on and 'side' for in. Bear with me :) For example, if you want to say "What is on your car" versus "What is in your car" and you wonder what the difference is, you can change it to be more clear with "What is on top of your car" versus "What is inside your car". Now hopefully you can figure out which meaning you want to convey. This assumes you know what "on top of" means versus "inside". As bluebottle pointed out, it starts to get messy with less 'pure' uses of these words. Examples: What's going on? (what is happening?) Are you putting me on? (are you trying to trick me?) Are you in? (are you joining me/us?) In a minute (it will be a minute before I'm ready)

Aha! There is no easy answer to this, but generally, imagine a closed box. Immediately outside the box, above, sitting, we call 'on the box'. Also placed against any of the 4 walls of the box, we call 'on the box'. There is another, smaller, space related to the box; 'in'. This separates the 'inside' from the 'outside'. The smaller space (the inside) is 'in the box'. You could catch a bluebottle inside this space; it would be in the box. From these classic, pure uses of 'in' and 'on' we can get hundreds of less literal meanings. What is in the newspaper? = what news is there? What is in this cake? = eggs, flour. Did you touch on this matter before? = did you briefly mention it?

They are prepositions, IN, ON, AT. https://www.englishclub.com/grammar/prepositions-at-in-on-time.htm https://www.englishclub.com/grammar/prepositions-place-at-in-on.htm