Preprosition

A preposition is a word which shows relationships among other words in the sentence.  The relationships include direction, place, time, cause, manner and amount.  In the sentence She went to the store, to is a preposition which shows direction.  In the sentence He came by bus, by is a  preposition which shows manner.  In the sentence They will be here at three o'clock, at is a preposition which shows time and in the sentence It is under the table, under is a preposition which shows place.

A preposition always goes with a noun or pronoun which is called the object of the preposition.  The preposition is almost always before the noun or pronoun and that is why it is called a preposition.  Thepreposition and the object of the preposition together are called a prepositional phrase.  The following chart shows the prepositions, objects of the preposition, and prepositional phrases of the sentences above.
 

Preposition	Object of the Preposition	Prepositional Phrase
to	the store	to the store
by	bus	by bus
at	three o'clock	at three o'clock
under	the table	under the table

Prepositional phrases are like idioms and are best learned through listening to and reading as much as possible.   Below are some common prepositions of time and place and examples of their use.

Prepositions of time:
at two o'clock
on Wednesday
in an hour, in January; in 1992
for a day

Prepositions of place:
at my house
in New York, in my hand
on the table
near the library
across the street
under the bed
between the books