A preposition is a word or set of words which link nouns, pronouns and phrases to other words in a sentence.

Prepositions are usually short words, and they are normally placed directly in front of nouns. In some cases, you’ll find prepositions in front of gerund verbs.

It also indicates location (in, near, beside, on top of) or some other relationship between a noun or pronoun and other parts of the sentence (about, after, besides, instead of, in accordance with).

A preposition isn't a preposition unless it goes with a related noun or pronoun, called the object of the preposition.

Generally speaking one can classify prepositions in three groups: prepositions of time, prepositions of place and other types of prepositions,  such as preposition of direction and preposition of quantity.

Example:

• I prefer to read in the library.

• He climbed up the ladder to get into the attic.

• Please sign your name on the dotted line after you read the contract.

• Go down the stairs and through the door.

• He swam across the pool.

• Take your brother with you.