on, in, at
How can I simple learn when I maust say "in" when "on" when "at". Have you, may be, some non oficialy rule when you need say every of this words?

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Ok, and what about: I will met him at midday? Is it correct?

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time http://www.englishclub.com/grammar/prepositions-at-in-on-time.htm
place http://www.englishclub.com/grammar/prepositions-at-in-on-place.htm

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There is a lot of different cases. 
I think I can give you a link with that rules, if you need it. 
Write me back, I'll find It later.