A preposition is used to link noun, pronouns and phrases to other words in a sentence. The word or phrase that the preposition introduces is called the object of the preposition. A preposition is used to indicate the temporal, spatial or logical relationship of its object to the rest of the sentence. Here are some examples:

The pencil is ON the desk.
The pencil is BENEATH the desk.
The pencil is leaning AGAINST the desk.
The pencil is on the floor BESIDE the desk.
He held the pencil OVER the desk.
He wrote with the pencil DURING class.

In each of the preceding sentences, a preposition locates the noun "pencil" in space or in time.

Here are some general rules regarding prepositions:

• It is permissible to end a sentence with a preposition.
• A preposition is followed by a noun.
• A preposition is never followed by a verb.
• Never begin a sentence with a preposition.
• A prepositional phrase always begins with a preposition and ends with a noun or pronoun called the OBJECT of the preposition.
• The subject of the sentence can never be part of a prepositional phrase.
• A verb can never be a part of a prepositional phrase.

There is a so-called “rule” about never ending a sentence with a preposition and it comes from Latin grammar. In Latin grammar, the word order of a sentence didn't matter; subjects and verbs and direct objects could appear in any sequence. However, the placement of prepositions was very important. A Latin sentence would quickly become confusing if the preposition did not appear immediately before the object of the preposition, so it became a stylistic rule for Latin writers to have objects always and immediately following prepositions. This Latin grammar "rule" meant that a sentence would never end with a preposition.

When English grammarians in the 1500s and 1600s starting writing grammar books, they tended to apply Latin rules to English, even though those rules had never been applicable before. I believe that they wanted to make English a more scholarly language, like Latin.

Here is a list of some prepositions: