A preposition is a part of speech that shows a relationship between another word and a noun phrase, pronoun, or other complement. Words such as “in,” “on,” and “under” can indicate place: “in the room,” “on the shelf,” or “under the bridge.” They can also express time, as in “in July,” “on Monday,” or “under pressure” in a more abstract sense.
The phrase headed by a preposition is called a prepositional phrase. In “The keys are under the cushion,” “under the cushion” tells us where the keys are, and “cushion” is the object or complement of the preposition. Prepositions can also express direction, manner, cause, possession, and comparison.
Learners often confuse prepositions with adverbs. In “She went inside,” “inside” has no object and functions adverbially; in “She went inside the house,” it introduces a noun phrase and functions as a preposition. English also has multiword prepositions such as “because of,” “in front of,” and “according to.”