Preposition, in grammar, is a word that introduces a word or phrase and connects that word with the word it modifies. In “The house beside the stream,” beside is a preposition. It has an object, stream. The phrase as a whole acts as an adjective modifying house.
There are only about 60 English prepositions. The most common ones include at, by, in, for, on, to, and with. Prepositions must have objects, the words or phrases they introduce. Thus, down is a preposition in the sentence “Mary fell down the well.” In the sentence “Mary fell down,” down has no object and is an adverb.
Some prepositions express clear meanings, such as time (before, during, following), or location (against, near). Other prepositions function mainly to express grammatical meanings, as in “The barking of the dogs kept me awake.” By definition, prepositions are single words, but there are multiword phrases of the form preposition-noun-preposition. These phrases function as a unit (by means of, in addition to, in spite of).
Prepositions also are used with nouns, adjectives, and verbs in an idiomatic sense–that is, the combination of words assumes a special meaning. For example, we may agree with, agree to, agree on, agree about, or agree among. We may have a preference for something, but choose one thing in preference to another. We may be doubtful of or about. A woman may be impatient with her secretary or impatient for the arrival of a friend. No rules explain the differences in usage.
It used to be said that one should never end a sentence with a preposition. However, this rule is now considered old-fashioned and never really described actual usage. Prepositions ordinarily precede their objects, but that order need not be followed. Most people recognize that “He knew what he came for” is a more natural expression than “He knew for what he came.”
In many of the world’s languages, such as Japanese, the equivalent of English prepositions come after their objects. Since these words come after rather than before their object, they are called _post_positions rather than _pre_positions.
