 They tell us where things are and when they happen(ed).  Common Prepositions: To, From, For, At, Beside, With, Without, Of, Between, In, Out, On, On top of, Under, Over, Above, Below, Beneath, Through, Before, After, During  See p. 515 in Writer’s Inc. for a list of prepositions  

 A prepositional phrase is a phrase that starts with a preposition (shocking, I know!). For the purposes of the SAT and ACT, prepositional phrases consist primarily of prepositions, nouns/pronouns, and adjectives. They do not contain verbs.  See p. 515 in Writer’s Inc. for examples of prepositional phrases

 At my house  During the movie  Between you and me  To my older sister *A prepositional phrase begins at the preposition and ends right before the verb (if there is one).

 The stack of books is sitting on the kitchen table.  One of the stories on the front page of the newspaper discusses the upcoming elections in great detail.  The train is crowded with people on their way home from school and work.  Sitting on the table are a peach and an apple.

 Prepositional phrases are frequently inserted between subjects and verbs on both the SAT and the ACT in order distract from disagreements, so whenever you don't immediately see an error in a sentence, it's a good idea to cross out all the prepositional phrases.  All information regarding prepositions and prepositional phrases came from