Prepositions are words placed before nouns or pronouns to indicate their position, time, or direction in relation to something else. Prepositions of time describe specific time periods like dates, days of the week, or when an event occurs. Examples include on, at, ago, before, later, after, until. Prepositions of place describe where something is located in relation to another thing, using words like on, in, at, below, above, under, behind. Prepositions of direction indicate something is headed toward a place, with words including to, towards, into, through, from, over.