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Okay, this is my theory regarding "in" and "on" prepositions by comparing those sentences below to each other

1."The house is on fire"

The "on" seems, to me, to indicate the state of the house, and it's on fire

2."The house is in good repair"
"I must put my affairs in order"

The "in" in each of the 2 sentences seem to also indicate the state of the objects. The house is in a repair state, and the affairs, in order state

And here's my question regarding this now. How do I exactly know when to use "on" instead of "in"to describe a state on an object or vice versa?

Thank you
Comments  
Here's my attempt:

I think of "on fire" as an idiom meaning that the house has caught fire, and at least some part of it is burning.

"In fire" would mean completely emerged in flames.

Anton

EDIT: Also see: http://www.EnglishForward.com/English/OnFireInFire/mldnr/post.htm
GuyperHow do I exactly know when to use "on" instead of "in"to describe a state on an object or vice versa?
By reading and listening to English as much as possible and remembering which preposition is used in each case. Write these expressions down and drill yourself on them until they become very familiar to you. Many choices of preposition are simply traditional, arbitrary uses that have become standard over many years. This is, of course, unfortuate for the learner, but true nevertheless.

If you do find any information that indicates that such expressions with in and on can be categorized in some logical way, thereby contradicting what I said above, please post it.

CJ