Prepositions link phrases, pronouns, and nouns to other words within a sentence.  Prepositions are known as references to location and time.  See the following examples:

The apple is on the counter.

The apple is beside the phone.

She ate the apple during the game.

Prepositional Phrases

Prepositional phrases often appear in sentences with prepositions.  They appear in the following form:  Preposition + Optional Modifiers + Noun, Pronoun, or Gerund.

Here are some examples:

Under (preposition) the (modifier) stove (noun)

According (preposition) to (modifier) Linda (noun)

Near (preposition) the (modifier) bookcase (noun)

Subordinate Conjunctions

There are also prepositions which function as subordinate conjunctions using words such as "As", "After", "Before", "Since", etc.  This type of clause has both a verb and a subject following it.  Here are some examples:

Before (subordinate conjunction) Anna (subject) took (verb) a shower

Until (subordinate conjunction) we (subject) meet (verb) again

As (subordinate conjunction) you (subject) wish (verb)

One exception to the subordinate conjunction is if you have a noun right after these following prepositions: "After", "As", "Before", "Since", and "Until".  When this occurs, you have a prepositional phrase.  Here are some examples:

After breakfast

Before noon

Since the divorce