PREPOSITIONS

What do prepositions do?
Prepositions are words (such as in and out, above and below, to and
from) that show the relationship between other words and phrases in a
sentence. (You'll find a list of the common prepositions at our
glossary entry for preposition.) Prepositions often show location
("under the table"), direction ("to the south"), or time ("past
midnight").

Are all prepositions single words?
No. In addition to the simple (one-word) prepositions, several word
groups (such as "in addition to" and "such as") perform the same
grammatical function. These word groups are called complex
prepositions. (You'll find a list of the most common ones at our
glossary entry for complex preposition.)

What is a prepositional phrase?
Prepositions aren't in the habit of standing alone. A word group with
a preposition at the head followed by an object (or complement) is
called a prepositional phrase. The object of a preposition is
typically a noun or pronoun: Gus put the horse before the cart.

For practice in recognizing prepositional phrases, try this exercise:
Identifying Prepositional Phrases.

What do prepositional phrases do?
Prepositional phrases add meaning to the nouns and verbs in sentences.
They usually tell us where, when, or how. A prepositional phrase may
do the work of an adjective and modify a noun: The student in the back
row began to snore loudly. It may also function as an adverband modify
a verb: Buster fell asleep during class.

To learn more about what prepositional phrases can do, see these two articles:

Adding Prepositional Phrases to the Basic Sentence Unit
Arranging Prepositional Phrases


Are we still expected to follow that old rule about never ending a
sentence with a preposition?
That's a "rule" (based on the etymology of "preposition" and a false
analogy to Latin) that you just don't have to put up with. As long ago
as 1926, Henry Fowler dismissed the rule about "preposition stranding"
as "a cherished superstition" ignored by major writers from
Shakespeare to Thackeray. In fact, he said, "the remarkable freedom
enjoyed by English in putting its prepositions late and omitting its
relatives is an important element in the flexibility of the language"
(A Dictionary of Modern English Usage).

To learn more about this false rule, see the article Is It Wrong to
End a Sentence With a Preposition?

Can a preposition ever function as another part of speech?
Yes. Certain prepositions (after, as, before, since, until) serve as
subordinating conjunctionswhen they're followed by a clause:

You better get out of town before sundown. (preposition)

Many people run out of ideas long before they run out of words. (conjunction)

Some prepositions (including about, across, around, before, down, in,
on, out, and up) also moonlight as adverbs (sometimes called
prepositional adverbs or adverbialparticles):

Beth walked up the driveway. (preposition followed by the object)

Beth looked up. (prepositional adverb modifying the verb looked)

For more information, visit our glossary entry for prepositional verb.

Why is it that English prepositions are often so baffling to students
of English as a second language?
We'll turn this one over to Ben Yagoda, author of When You Catch an
Adjective, Kill It(Broadway, 2007): "Prepositions are insane!
Sometimes they make no sense--it's just a case of memorizing them."

For practice in using prepositional phrases effectively, visit these
articles and exercises:

Identifying Prepositional Phrases
Adding Prepositional Phrases to the Basic Sentence
Expanding Sentences With Prepositional Phrases
Arranging Prepositional Phrases
Sentence Building With Prepositional Phrases
Sentence Combining With Simple Modifiers

Source --http://grammar.about.com/od/basicsentencegrammar/a/notesonprepositions.htm