prepositions

A preposition is a word governing, and usually coming in front of, a noun or pronoun and expressing a relation to another word or element, as in:
She left before breakfast.

There is one very simple rule about prepositions. And, unlike most rules, this rule has no exceptions. The preposition is followed by a "noun". It is never followed by a verb.
By "noun" we include:
noun (dog, money, love)
proper noun (name) (Bangkok, Mary)
pronoun (you, him, us)
noun group (my first job)
gerund (swimming) hat did you come for?

Prepositions of Place: at, in, on
In general, we use:
at for a POINT
in for an ENCLOSED SPACE
on for a SURFACE

Prepositions of Time: at, in, on
We use:
at for a PRECISE TIME
in for MONTHS, YEARS, CENTURIES and LONG PERIODS
on for DAYS and DATES