on vs in

They're (mostly, not always) prepositions (used in prepositional phrases (on the table, in the hall, etc), which will help you a little more. However, I also think regional difference comes into play with these two. Some Americans will use 'in' some 'on' when it comes to 'place', but UK and US differences come in too.

I'm in Hampton Street (UK)
I'm on 5th Street (US)

So when and where to use them will come down to your region, as well as grammar books. But there are going to be differences, which doesn't help too much, I'm afraid.

Agree with Fallen.

'In' usually relates to being 'inside' or surrounded/enveloped by something be it a house, a room, a bath, trouble, mood, fog, heaven, confusion, a crowd, a gang, etc..

'On' just doesn't work or sound right in such situations and the simplest way to solve this issue may be to ask which option makes the most sense to you.

Reminds me of the old, old joke observing English-speaking viewers watched programs "on" TV, and German speakers watched shows "in" the TV set.

So it goes that when the German was asked what he thought of sex on television, he replied, "It is okay as long as they don't fall off."

Hardly funny these days when TVs are waffer-thin and hung on walls like a framed painting.

I agree that there's a regional aspect to it. I hadn't heard this construction until I was in my 30s, but there are pockets of the population that say "on line" as opposed to "in line", as in: "We waited on line for two hours." I would say, "We waited IN line for two hours."

There are some hard-line grammatical uses for on vs. in but there are idiomatic uses also. If you have specific examples, we could help you out with those.