Prepositions show the relationship between nouns or pronouns and other parts of a sentence. There are three main types of prepositions: place, time, and movement. Prepositions of place indicate location, such as "above" or "under." Prepositions of time specify when something occurs, like "at night" or "in the morning." Prepositions of movement show direction, for example "to school" or "into the river." The document provides examples of common prepositions for each category and explains how prepositions function in sentences.