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A Few Essentials

Some of the comma rules target "essential"  and "nonessential" elements.  Sometimes, the terms restrictive and non-restrictive are used, but the concept is the same.  Textbook definitions aren't always helpful in defining what essential means, so let me give it a try.

The comma rules say to put commas around the nonessential phrases and clauses to show that they are nonessential, non-crucial elements because they don't lower (I often say limit) the number of possible meanings.

When a phrase or clause is essential, which means necessary or crucial, it is essential because
it lowers the number of possible meanings of the word(s) it modifies or renames.  Usually, it reduces that number to just one possibility, but it doesn't have to.  My best guess is you need an example at this point. Please take a look at this sentence with an essential participial phrase:

That man at the football game wearing clown shoes and makeup is my weird uncle.

Context is important here.  Since the setting is a football game, we can assume that the man is one of many men who are attending the game.  Consequently, just saying "that man" is pretty general:  it could be any man there.  Unless your weird uncle said to his clown friends, "Hey, you Bozos, let's go to the game!" and they all piled in their little clown car and are now sitting in the stands, then we can assume he's the only clown there and "wearing clown shoes and makeup" has lowered the possible meanings of man to just one.  Consequently, this participial phrase is essential because it lowers the number of possible men, so it is not separated from the rest of the sentence with commas.  See, how I can really explain concepts once I get warmed up?  Questions?

What phrases and clauses can be essential and nonessential?

An excellent question!  Participial phrases (like the one in the example), appositive phrases, and adjective clauses can be essential or nonessential. Noun clauses that act as appositives can also be essential or nonessential.

I went to some length to explain what these are on my grammar page.  Use the links below for more information.

We will do some exercises in class, but let me show a few examples here.

Most appositives and appositive phrases are nonessential since it's more common to rename a specific noun with an appositive that gives more information but does not help lower the number of possibilities. Appositive information

A nonessential appositive phrase:  I adore Edgar Rice Burroughs, the author of the Tarzan books
ERB lowers the number to one person, so the appositive phrase can't help lower the number of possibilities any more than that, is nonessential, and set off by the comma. 

An essential appositive (no phrase):  My first-cousin Jane was named after Tarzan's Jane.
Jane renames first-cousin.  In this context the writer has more than one first cousin, so Jane lowers the number of possibilities to one, is essential, and requires no commas.  Context: Imagine that example in a passage about cousins and their names.

A nonessential appositive (no phrase):  My only first-cousin, Jane, was named after Tarzan's Jane.
Adding only allows first-cousin to limit the number to one, and Jane can't do more than that.  Consequently, Jane is set off by commas as nonessential.  Of course, it would be more common to write "Jane, my only first-cousin, was..." making first-cousin a nonessential appositive.

Adjective clauses are a little like participial phrases since participles look like verbs and act like adjectives, and adjective clauses, obviously, act as adjectives.  Of course, clauses have a subject and a verb, and subordinate clauses, like the adjective clause, have a word that begins the clause that helps you identify it:  who, whom, whose, which, and, that, the relative adjective whose, and the relative adverbs where and when may begin adjective clauses. Some of these are part-time interrogative pronouns (used to ask questions) and don't always introduce adjective clauses.  More information.

Nonessential adjective clause:  Thomas Edison, who "invented" the light bulb, actually had a team of scientists helping him. 
Thomas Edison lowers the number of possibilities to one, so the adjective clause is nonessential and set off by commas.

Essential adjective clause:  The scientist who invented the light bulb was Thomas Edison.
The word scientist lowers the possibilities to millions of living and dead scientists, but the adjective clause lowers the number to the one man who gets the credit.   Consequently, it's essential, and no commas are needed.

Typically, adjective clauses that begin with that are essential, and ones that begin with which are nonessential since readers pick up on the connotations of these words.  Clauses that begin with which seem like an afterthought rather than essential information.

Essential participial phrase:  The book covering the vent explains how uncomfortable the library is.
Nonessential participial phrase: President Obama, depressed by the vote in the Senate, closed his eyes.
By now, you should be able to tell me why these are essential and nonessential. More participial information.

A noun clause as a nonessential appositive:  His only idea, that we serve sausage in the concession stand, didn't fly with the administrators.  More noun clause information.

A noun clause as an essential appositive:  His idea that we serve sausage in the concession stand didn't fly with the administrators.

The only difference between these two clause examples is the noun they rename.  In the first example, only limits the possibilities to one idea, so the noun clause acting as an appositive is nonessential. In the second we need help to lower the number of possible ideas.

Tricky stuff

You can force an appositive phrase to precede the noun it renames: 
The inventor of the light bulb, Thomas Edison is world famous.
It's still nonessential and set off by a comma, but it's out of its usual spot after the noun it renames.

A different rule covers participial phrases when they come first in a sentence.  Introductory elements (stuff that comes before the subject and verb combo of an independent clause) are followed by a comma. You don't have to know if it's essential or nonessential in this case.
Working in the dark, Edison created the light bulb.