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Commas: Three Sentence Uses

Beginning Phrases

Commas are used to set apart two halves of a compound sentence, but they also come in handy for setting apart descriptive or non-essential information in a sentence. A comma can indicate a pause after an introductory phrase. A pair of them may also separate an idea from the main ideas of your sentence.

Introductory phrases establish details that are not directly related to the main idea of the sentence. In some cases, when you have an introductory phrase that is one word in length, you may opt to leave the comma out. However, if unsure, set off descriptive introductory words with a comma. 

A good rule of thumb is that if you have a prepositional phrase as your opener, you probably need to set it apart from the main action of the sentence.
In the 1950s, rock and roll music was born!
Across town, they have a lovely restaurant.
Through careful reflection, I was able to reach a decision.

Each of those introductory phrases is set apart by a comma because it describes something related to the main point of the sentence before the main subject and verb are introduced.
When a descriptive phrase leads away from the main action of the sentence, however, no commas are needed.

Rock and roll music was born in the 1950s.
They have a lovely restaurant across town.
I was able to reach a decision through careful reflection.

Appositives

Appositives are nouns or descriptive phrases set side-by-side with another noun to help to identify or explain. For example a noun and a qualifying description:

My cat, a beautiful Turkish Angora, loves to sleep on the bed.

"A beautiful Turkish Angora" is not integral to the main idea of the sentence, but it does help to describe the cat. Therefore, we set that info apart.

You will almost always use two commas, one on each side of the appositive, to set apart these kinds of descriptive or qualifying phrases.

There's an interesting rule of thumb with commas and appositives when you have a proper term and a brief definition next to it.
 It is customary to put a set of commas to separate a definition that comes after the formal term. But when the definition precedes the formal term no commas are necessary. For example:

Abraham Lincoln, the sixteenth president, was shot by John Wilkes Booth, an actor.

However, if the descriptions lead up to the proper names, no commas are needed because the names are more important than the definitions and should be part of the sentence proper.

The sixteenth president Abraham Lincoln was shot by the actor John Wilkes Booth.

Commas and Coordinating Conjunctions

Compound sentences joined by a conjunction require a comma before that conjunction. However, just because you have a word like "and, or, so, but, etc." does not always mean you must have a comma.

When you place a comma in your sentence always know what it is doing and why you have placed it in a specific spot. Sometimes it is just as important to know when not to use one. For example:

 Incorrect: Shakespeare mastered the art of comedy, and inspired countless generations to laugh.
Correct: Shakespeare mastered the art of comedy and inspired countless generations to laugh.

A pair of words joined by a conjunction that links back to the same subject makes a complex sentence. You would not want a comma to 
come before "and" in the above example because it is simply linking a pair of works like "ham and cheese" or "run and leap."

When dealing with lists of actions, things, or ideas, remember that two items joined by a conjunction like "and" do not need to be set apart by commas. Only lists of three or more.

Correct: This summer's weather is hot, oppresive, and deadly.
Correct: This summer's weather is hot and oppressive.

However, if you have a compound sentence with a complete set of subject and verb on either set of the conjunction, then you DO need a comma. Example:
Correct: Shakespeare mastered the art of comedy, and he inspired countless generations to laugh.