A preposition is a word that begins a prepositional phrase and shows the relationship between a noun or a pronoun to another word in the sentence. A preposition must always have an object. A prepositional phrase starts with a preposition, ends with the object, and may have modifiers between the proposition and object of the preposition. (The OBJECT of a sentence receives the action from the subject-verb. The SUBJECT performs the action/verb).
“Anywhere a mouse can go”: Here is a list of common words that can be used as prepositions: about, above, across, after, against, along, among, around, at, before, behind, below, beneath, beside, besides, between, beyond, but (when it means except), by, concerning, down, during, except, for, from, in, inside, into, like, near, of, off, on, out, outside, over, past, since, through, to, toward, under, until, up, upon, with, within, and without.
These words can be used as other parts of speech. What part of speech it is depends on how it is used in that sentence. Many of the common words used as prepositions can be used as adverbs. Words are prepositions if they have an object to complete them. To decide which it is, say the preposition followed by whom or what. If a noun or a pronoun answers the question, the word is a preposition.
Example: The boy stood up and ran down the street. Up what? There is no object; therefore up is not a preposition. Down what? Street answers the question; therefore, down is a preposition. Down the street is the prepositional phrase starting with the preposition down and ending with the object street with a modifier the in between.