Prepositions are another relatively small and stable class of words. There are around 50 in regular use in English. A preposition functions to identify relationships: of time and space, agency and instrumentality.

There is also a set of compound prepositions that are formed with from multiple prepositions, adverbs, adjectives, nouns and conjunctions.

Prepositions are used in English where a case structure might be used in other languages. In a case structure, nouns inflect in a declination, a falling away. The word case comes from a Latin term casus which is derived from a verb cadere and means to fall.

Prepositions occur only as elements of prepositional phrases, which are a noun-phrase introduced by a preposition. Prepositional phrases can be an element of a noun-term, an adjective-term and an adverb-term and as such, they can be part of any statement component except the finite verb.

Prepositions of time are can also function as conjunctions in English and the same effect can be achieved either by the use of a preposition or a conjunction. The only difference is that the conjunction while is used instead of the prepositions during and throughout.