First, let’s begin with the definition of a preposition. A preposition is simply any word placed before a noun or pronoun to create a modifying phrase for another word in the sentence. A prepositional phrase begins with a preposition and ends with a noun or noun equivalent, called the object of the preposition. The prepositional phrase nearly always functions as either an adverb or an adjective. For example, everyone, or nearly everyone, has heard the sentence, "The road to hell is paved with good intentions." Here, "to hell" is the prepositional phrase and is functioning as an adjective modifying the "road." The prepositional phrase "with good intentions" is functioning as an adverb because it is modifying the word "paved." Remember, an adjective is a modifier that answers the questions, "What kind of?" or "Which one?" while an adverb is a modifier that answers the questions, "Why?" "When?" "How?" "How much?"

When functioning as an adjective, the prepositional phrase nearly always appears immediately following the noun or pronoun it is modifying. For example, in the sentence, "Variety is the spice of life" if we ask which spice or what kind of spice, we get the answer, "the spice of life." The phrase answers at least one of the adjective questions. Additionally, the prepositional phrase "of life" is functioning as an adjective because it cannot be moved to any other place in the sentence and have it make sense; "of life" is wedded to spice. However, prepositional phrases functioning as an adverb it is usually able to be moved. For example, the prepositional phrase, "by its fruit" is adverbial because it can be put into a sentence to read, "You know an apple tree by its fruit" or the phrase can be moved to the beginning of the sentence to read, "By its fruit, you know an apple tree." Either way, the sentence still makes sense. If we ask the question of "you will know an apple tree how?" we get the answer of "by its fruit." The prepositional phrase answers at least one of the adverb questions.

The following words are all prepositions:

As you can see from the list, some prepositions are more than one word long. There are more "compound prepositions" (meaning having more than one word), but I have only included the most commonly used ones here.

In writing, you generally want to avoid overusing prepositions and prepositional phrases because they tend to slow the flow down and the reader can easily become bored. Also, using too many prepositional phrases in one sentence will change your writing from active voice to passive. They do this because it is nearly impossible to have more than one prepositional phrase in a sentence and not have a form of the verb "be." The "to be" verbs are the ones responsible for turning active to passive.

Note: There are time when the word "to" is not a preposition but rather an infinitive marker. Such as in the phrase, "to sleep, perchance to dream" the word "to" signals the infinitives of "sleep" and "dream." For more, see the definition of "infinitive."