A preposition shows the relationship between a noun or pronoun and another noun or pronoun.
the cat under the fence
the cat between the fence and the house
everyone except the girl in the blue dress
a letter about us
The italicized words in the preceding phrases are prepositions; the underlined words are objects of the prepositions. When the object is a pronoun, remember that the pronoun should be in the objective case.
How do you recognize a preposition? It's sometimes not easy. Prepositions aren't as obvious as nouns and verbs. Look for a word that establishes a certain kind of relationship with another word. For example, in the previous phrases, how is cat related to fence? The cat is under the fence. How is everyone related to the girl? The girl is left out of the group everyone. How is girl related to dress? She is in it.
Some prepositions, called compound prepositions, are made up of more than one word, such as according to, because of, in front of, instead of, in spite of, and next to.