Prepositions (prepositions, referred to as prep) are one of the most active parts of speech in English, words that connect the subject and the predicate. A preposition is a function word used to express the relationship between words and words, and between words and sentences. It cannot be used as a separate sentence component in a sentence. A preposition is usually followed by a noun pronoun or other part of speech equivalent to a noun, a phrase or a clause as its object. A preposition and its object form a prepositional phrase, which serves as an adverbial, predicate, complement or object of a preposition in a sentence.