Judging by the comments on the “When to use ‘on’ and when to use ‘in’” post, I wasn’t the only one to get out of school with less than a complete grasp of what a preposition is or does.
When I was in high school, I never could pick out prepositional phrases. Oh, I memorized the lists of prepositions like in, on, up, with, to, and from. I just couldn’t figure out how they worked in a sentence. After all, some of the words in the list could also be used as adverbs: I went to the river and jumped in.
A word isn’t anything until it is used in a sentence.
It wasn’t until I started teaching English that I finally “got” prepositions.
Prepositions are joining words. They join something to a noun (or pronoun). George V was King of England. The preposition “of” joins the noun “King” to the noun “England.” The joining adds information to the noun “King.”
The boy played in the water. The preposition “in” joins the verb “played” to the noun “water.” The joining conveys where the playing took place.
The boy in the water is my brother. Here the preposition “in” joins the noun “boy” to the noun “water.” This joining identifies a particular boy, distinguishing him from a possible “boy on the beach” or “boy in the boat.”
Prepositions are said to “govern” nouns (or pronouns). The usual position of a preposition is in front of the noun it governs.
A “prepositional phrase” is the preposition and its noun, plus all the words that come between.
Sometimes a prepositional phrase consists of just the preposition and the noun (or pronoun) that it governs: This book is by Tolstoy. (preposition “by,” noun “Tolstoy”)
Sometimes the phrase has several words between the preposition and the noun governed: We went to the new outlet mall. (preposition “to,” noun “mall”)
Prepositional phrases function as parts of speech.
The boy in the water is my brother. (“in the water” tells more about “boy.'” The phrase functions as an adjective describing “boy.” He’s the in-the-water boy.)
The dog is swimming in the water. (“in the water” tells where the swimming is taking place. The phrase functions as an adverb modifying “swimming.”)
Although the preposition usually comes before the noun it governs, English permits us to place the preposition at the end of a sentence. (Purists abhor this construction, but it is very natural in English.) Ex. Whose house do you want to meet at? (The prepositional phrase is at whose house.)
By the way, it is this practice that is contributing to the decline of “whom” as the object form of “who.” The ear recognizes the need for “whom” when “to” precedes it, but not when the “to” is far removed: Who do you want to speak to? as opposed to To whom do you wish to speak?
As for wanting rules that will explain every prepositional expression such as in the army, on the team, at the hour, or in a month–save your energy. They don’t exist.