A preposition is a hook for a noun or pronoun to hang on. It usually
precedes the noun or pronoun which hangs, or depends upon it, as
indicated by its name which is derived from the Latin pre-before and pono-I place.
John is behind the press.
I shall work until Sunday.
A preposition shows the relation of a noun or pronoun used as its object
to some other word or words in the sentence or, as it has been otherwise
stated, makes the noun or pronoun to which it is joined equivalent to an
adjective or an adverb. The expression "John is behind the press" is
equivalent to an adjective describing John. That is, he is "John
behind-the-press." Prepositions are governing words and the words
governed by or depending on them are always in the objective case.