Very young children write simple sentences. 
The above is a simple sentence with one subject and one verb.  This is the way that we all begin to write. 

Slightly older children combine them, and they make compound sentences. 
As we become older, we stick two or more of these simple sentences together with a comma and the conjunction "and".

Phrase (noun) A word or group of words forming a unit and conveying meaning

Roget's II: The New Thesaurus, Third Edition. 1995.

Most students write the above two types of sentences, but many are limited to ONLY writing these two types.   
The purpose of this unit is to encourage students to grow in their writing, to experiment with advanced sentence structure that will give their writing more depth and variety.   We will do this by studying and practicing the use of phrases. 

What is a Phrase?
The phrase is a PART of the sentence, but not a necessary part. 
It can be removed and the sentence should still make sense.   

As students grow in sophistication, their sentence structures become more highly developed.   (This is a prepositional phrase, which begin with words like after, before, under, over, and if.)

Learning more every year, these writers become familiar with superior techniques and methods.  (This is a participle phrase, which begin with verbs.  Usually they end in -ing.)

Older students, generally more advanced writers, develop more complex approaches to writing.  (This is an appositive phrase, which contains a noun that renames another noun.)

A student, dedicated and focused, will try hard, and he will succeed.

(Theses are adjectives out-of-order.)

The Test - Can it Be Moved? 
Since a phrase is a piece of additional material,
the writer should be able to move it to at least one other spot in the sentence.

Shaking uncontrollably, the hunter could not get the arrow to stay on his rest. 

Shivering in the darkness, the girl felt great relief from fear. 

Hitting their turbo boosts, the cops caught right up with him. 

Not expecting it, Roxanne let the ball come right back over. 

Not knowing what the other guy was going to do, he decided to bring the girl down to the station. 

The hunter, shaking uncontrollably, could not get the arrow to stay on his rest. 

The girl
, shivering in the darkness, felt great relief from fear. 
The girl felt great relief from fear
, shivering in the darkness

The cops
, hitting their turbo boosts, caught right up with him. 

, not expecting it, let the ball come right back over. 

He decided
, not knowing what the other guy was going to do, to bring the girl down to the station.   
He decided to bring the girl down to the station
, not knowing what the other guy was going to do.     

The variation that phrases can bring to sentences is demonstrated below. 

This man's name is Dwain and he grew up in the Rocky Mountains.  In Colorado, he stayed far away from civilization all his life.  He lived with his two younger brothers and his father.  They all have the same brown shaggy hair, full beards and brown eyes.   

High School Student

Sentence Length Analysis

Sentences that have nearly the same length become monotonous. 

Tired from his romp, Wilbur lay down in the clean straw.  He closed his eyes.  The straw seemed scratchy - not as comfortable as the cow manure, which was always delightfully soft to lie in.  So he pushed the straw to one side and stretched out in the manure.  Wilbur sighed.  It had been a busy day - his first day of being terrific.  Dozens of people had visited his yard during the afternoon, and he had had to stand and pose, looking as terrific as he could.  Now he was tired.  Fern had arrived and seated herself quietly on her stool in the corner. 

E.B. White Charlotte's Web

Sentence Length Analysis
------------------- 19 

Varied sentence length  improves writing. 

Notice how the longer sentences at left indicate busyness, while the shorter ones indicate tiredness. 

There is a fifth dimension beyond that which is known to man.  It is a dimension as vast as space and as timeless as infinity.  It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of man's fears, and the summit of his knowledge.  This is the dimension of imagination.  It is an area that we call the twilight zone. 

Sentence Length Analysis

Using a short sentence after a longer one gives the reader a little break.

It all began in the cold.  It had been cold all week in Washington.  The early Thursday afternoon the snow came.  The winds blew in icy, stinging gusts and whipped the snow down the frigid streets.  Washingtonians do not know how to drive in the show: they slide and skid and spin their wheels in panic.  By six o'clock traffic had stopped all over town.  People abandoned their cars in snowdrifts and marched grimly into the gale, heads down, newspapers wrapped around their necks and stuffed under coats.  And still the snow fell and the winds blew. 

Sentence Length Analysis