The short version is in this post; a more detailed explanation which includes some exercises to practice can be downloaded here (prepositions-at-in-on.pdf). The answers to the exercises are available here.
Prepositions of place
In general we use:
- at a POINT
- in an ENCLOSED SPACE
- on a SURFACE
Examples:
- Peter is waiting for you at the cinema
- Do you live in Italy?
- Someone has spilt wine on this menu
Prepositions of time
In general we use:
- at a PRECISE TIME
- in a PERIOD OF TIME
- on a DATE or DAY
Examples:
- We usually eat lunch at 1 p.m.
- Are you going on holiday in June?
- Where will you be on your birthday?
