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The most common prepositions of location are
(See: In, On, At)
in front of
in back of
on top of
These prepositions are most commonly followed by "the" and a noun.
Both the speaker and the listener likely know which object is being referred to.
The keys are on the table. (We both know which table.)
The post office is next to the bakery. (Not: next to bakery)
Occasionally, prepositions can be used with "a/an."
This usually indicates that the speaker knows of the place, but the listener does not.
I live by a river. (You probably don't know which one.)
I live by the river. (You know the river I'm talking about.)