Separating items in a series signals to the reader that the items are "like" in some respect. The series may be a series of adjectives describing something, a series of things to pick up at the market, or a series of adjective, infinitive, or prepositional phrases.  A series of two or more items should be separated by commas; however, there is some debate about whether the last item in a list of three items should take a comma.  Many grammarians would argue in this situation, leaving out the last comma is acceptable.  However, MLA format rules do require a comma before the last item in a series of three or more elements.

examples:

Felix dislikes spinach, broccoli, green beans, and brussels sprouts—any vegetable that's green.

Doing the dishes, washing the clothes, and mopping the floor are the chores I hate most.

Finding healthy, appetizing fast food is difficult.

Note: Sometimes a series seems to be "like" when it is not.  In this situation, a comma should not be used between the items. For example,

She left her beautiful black leather handbag on the bus.

Haunted by the memory of last year's painful knee surgery, she protected her knee with a strong brace during the tennis match.

A useful way to tell if items are "like" is to try placing the word "and" between every item; if the sentence makes sense, then the items are like; if not, then they are unlike items and should not be separated by commas.

Two or more independent clauses joined together need strong punctuation: a semi-colon, or a comma and a coordinating conjunction.  This punctuation tells the reader that the clause could stand on its own as a complete sentence. Writers join clauses together to show a close relationship between ideas and to vary the rhythm in the sound of their sentences. And the coordinating conjunctions—and, nor, for, but, yet, and so—also help writers identify the relationship between ideas and create coherence in their writing (take a look at the module on coordination and subordination for more information about using conjunctions effectively). Without adequate punctuation, the sentence will be a "run-on sentence": if no punctuation is included between independent clauses, the error is a "fused sentence," if the coordinating conjunction is provided and the comma is omitted, the error is a "comma splice" (for more detail, explore the module on run-on sentences).

examples:

Felix does not like spinach, nor does he like broccoli.

We will be going to the mountains again this summer, but this time we will bring mosquito repellant.

After graduation, Maya is going to travel throughout Southeast Asia for a year, and I will be staying home flipping burgers and going to school.

 

A non-essential element is information that is added to the core of a sentence to add further information—information that is useful, but not really essential to understanding the basic assertion.  To signal the reader that a word, phrase, or dependent clause is non-essential information, writers set these elements off by commas before and after the addition.  The information is thus enclosed by commas. If the phrase occurs at the beginning or end of the sentence, only the comma separating the phrase from the rest of the sentence is needed.

examples:

She is, as you can probably tell, pretty nervous about public speaking.

As you can probably tell, she is pretty nervous about public speaking.

Once upon a time, before television, before computers, people read books for fun.

Jake brought his best friends from school, John and Alex, home with him for Thanksgiving dinner.

Note: In the last example above, the names of Jake's friends are non-essential information: Jake has only two best friends.  However, if the names are essential to identifying the person, the names should not be set off from the sentence.  Consider how the meaning changes with the presence or absence of enclosing commas in the following sentences:

My sister Maria is pretty fun to hang out with.

My sister, Maria, is pretty fun to hang out with.

In the first sentence, because the name of the sister is included as necessary to the meaning of the sentence, the speaker could have more than one sister—this particular sister is fun to hang out with. In the second sentence, because the name of the sister is non-essential information, the speaker must have only one sister—my sister, whose name, by the way, is Maria, is fun to hang out with.

Introductory Clauses:  When beginning a sentence with an introductory clause (as in this sentence), a comma is typically necessary to signal the reader as to when the introductory phrase or clause is complete. 

examples:

As you can see, the problem is much bigger than we imagined.

Because we have had so much rain this year, construction of the new highschool will take an additional six months.

Jake brought his best friends from school, John and Alex, home with him for Thanksgiving dinner.

Watch out, however, for clauses used as the subject of a sentence; one should never separate a subject from the verb of the sentence:

example:

What she has always wanted is now within her reach.

Referring to what others have said shows that you are participating in an ongoing conversation about a topic.

Introductory words:  Sometimes a sentence begins with a single word that needs to be set off from the rest of the sentence.

mild exclamations or interjections:

Hey, I thought you were going to study tonight instead of come with us to the movies.

Ok, then show me how you do it.

No, I don't want pineapple on my pizza.

a name in a direct address:

Miguel, do you want another cup of coffee?

a sentence adverb: these are special transition words that always take a comma, or if they occur in the middle of a sentence, they are enclosed by commas. Examples of sentence adverbs include moreover, however, nevertheless, furthermore, frankly, sadly, and mercifully. Never use a sentence adverb to connect two independent clauses unless you use a semi-colon before the sentence adverb.

I would love a new plasma television; however, I cannot afford one.

Sadly, she has the flu and will have to skip the concert.

Note: Grammarians disagree about the word "hopefully" used as a sentence adverb.  Most hate this construction, arguing that it is ambiguous—the writer could mean a hopeful prediction, or "I am hoping."

Introductory Phrases: 

Introductory phrases add information to the sentence and create interest in and anticipation for the information to follow.  All phrases should be set off from the rest of the sentence by a comma with two exceptions: short prepositional phrases do not need a comma unless it is necessary for clarity, and introductory appositive phrases that are essential to the sentence should never be separated from the subject by a comma.

examples:

To help him understand how an internal combustion engine works, I drew a diagram.

In San Francisco one finds plenty of good restaurants.

At the restaurant we all ordered burgers and fries.

Without a doubt, she is the nicest person I have ever met.

The acclaimed critic Roger Ebert has been ill recently, and unable to host his television program, Ebert & Roeper.

 

What follows reads a bit like a laundry list of comma use rules, but unfortunately or fortunately (depending on how we look at it), many conventions for the presentation of dates, names, times, citations, and quotations have risen up over time.  Troublesome they may be, but these conventions allow us to quickly share and process information.

. . . Use a comma to separate a city from a state, and after the state before continuing with the rest of the sentence:

I have lived in San Francisco, California, for most of my life.

. . . Use a comma to separate the day of the month from the year, and after the year before continuing with the sentence:

The twins were born in June, 1979, one month before my 30th birthday.

. . . Use commas to surround titles or degrees:

Professor Pete McSeed, Ph.D., received his doctoral from Amherst College a year before taking a teaching job at Yale.

. . . Commas used with quotations are a bit tricky. When introducing a quote with a short introductory phrase like "He said," "Twain argues," or "As Huck puts it," the phrase should be set off from the quote by a comma.  However, after an independent clause, a colon should be used instead.

In The Adventures of Huckleberry Finn, Huck says, "All I say is, kings is kings, and you got to make allowances. Take them all around, they're a mighty ornery lot. It's the way they're raised."

Huck embraces life in the wild to the extent that a raft moving down a river can provide all the home he really needs: "We said there warn't no home like a raft, after all. Other places do seem so cramped up and smothery, but a raft don't. You feel mighty free and easy and comfortable on a raft."

It is also important to note that a comma used after a quote to separate it from the rest of the sentence goes inside the closing quote.

Twain's short story "Hunting the Deceitful Turkey," is wildly funny.

. . . Use a comma to emphasize a shift or to cause the reader to pause before an important contrast or point of emphasis.

He was angry, furious even.

Tears were streaming down her face because she was happy, not sad.

You are going to make the cake for the party, aren't you?