In the most broad and simple terms, a preposition is a word that helps to tie or relate two other words together. Prepositions help you understand relationships such as place and time. There are approximately 150 prepositions in the English language. Prepositions can be tricky. They are often used extraneously and put into sentences in an incorrect position.
You likely use the words "to," "of," "for," "at" and "in" countless times in a day. You see them in print a number of times as well. Without these words, many sentences would lose their meaning. For example, if someone asks where your sister is going and you say, "she's going the store," you've left out a preposition that explains the relationship of space. The correct way to say what you mean, "she's going to the store" helps the listener understand that she is moving in the direction of the store. "To," "of," "for," "at" and "in" are arguably the most commonly used prepositions.
While many common prepositions relate words concerning space ("below," "inside," "near" and "under") they also help connect words concerning time. The prepositions "after" and "before" are two such words. When you explain an event that is after another one, the listener is able to have an understanding of order to your list.
There is no official ruling of what the most common prepositions are. For example, at Towson University, the most common prepositions include, but are not limited to, above, about, after, against, across, along, among, around, because of, except, during, through, past, over, until, up, with, since, past and regarding.
Because there are over 100 prepositions in English, memorizing each one may be taxing; it's easier to recognize a preposition based on its definition, sentence placement and usage. Remember that a preposition displays a relationship of time or space (and sometimes both), and that it is used to relate a sentence's nouns and pronouns to their objects. If you're not sure whether a sentence can be ended with a preposition, ask yourself if the sentence will still make sense without it.