A is a word that can be combined with a noun or pronoun to form a phrase that tells something about another word in the sentence.

Preposition

Explanation:

A Preposition is a word that shows relationship between noun and pronoun in a phrase. It helps to understand parts of speech fit together in a sentence.  

Prepositions play a major role in deriving complete meaning out of a sentence.

Example:

There are some books on the table (On- Preposition)

She sits in the car.                              (In- Preposition)

the anwser is went creeping down the river again, across the wide lonely marshes to the sea.

i longed to go with them out into the world to seek my fortune in foreign lands—africa, india, china and peru!

and, dreaming of the lands i had never seen, i'd sit on there, watching till they were out of sight.