In a sentence, a preposition is a word that shows relationship between a noun/pronoun and a verb, or two nouns/pronouns.
1. Kasim helped the old man go across the road.
2. The police man commanded the suspect to lean against the wall.
3. He hid behind the car to avoid being spotted.
In sentence 1, the preposition across links the noun ‘road’ and the verb ‘go’.
In sentence 2, the preposition against links the noun ‘wall’ and the verb ‘lean’.
In sentence 3, the preposition behind links the noun ‘car’ and the verb ‘hid’.
The most common prepositions
about, above, across, after, against, along, among, around, at, before, behind, below, beneath, beside, between, beyond, but, by, despite, down, during, except, for, from, in, inside, into, like, near, of, off, on, onto, out, outside, over, past, since, through, throughout, till, to, toward, under, underneath, until, up, upon, with, within, and without.