A preposition is a word that connects its object (a noun, a noun clause, or a pronoun that follows the
preposition) to another word in the sentence. A preposition is positioned before its object, typically
indicating a relationship based on time, space/location, or direction between the object and the rest of
the sentence:

          To garner more attention, Pauline placed the dead dog on her head.

          “I need to get to the finish line before Sanchez,” gasped Juan.

          (In the first sentence, on expresses a relationship of location: It tells where Pauline placed the
dead dog. In the second sentence, before describes a relationship based on time: It tells when
Juan would like to reach the finish line. The object of on is head; the object of before is
Sanchez.) 

     Prepositions can also express other relationships as well. For instance, a preposition such as
concerning shows a relationship of logic. Prepositions such as without and except show a relationship
of exclusion.

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