In a Time magazine essay titled "In Praise of the Humble Comma," Pico Iyer nicely illustrates some of the various uses of punctuation marks:
Punctuation has a point: to keep up law and order. Punctuation marks are the road signs placed along the highway of our communication--to control speeds, provide directions, and prevent head-on collisions. A period has the unblinking finality of a red light; the comma is a flashing yellow light that asks us only to slow down; and the semicolon is a stop sign that tells us to ease gradually to a halt, before gradually starting up again.
You probably already recognize the road signs of punctuation, though now and then you might get the signs confused.
Probably the best way to understand punctuation is to study the sentence structures that the marks accompany. Here we will review the conventional uses of punctuation in English.
We'll consider four main guidelines for using commas effectively. But keep in mind that these are only guidelines: there are no unbreakable rules for using commas-or any other marks of punctuation.
1. Use a Comma before a coordinator
Use a comma before a coordinator (and, but, yet, or, nor, for, so) that links two main clauses:
"The optimist thinks that this is the best of all possible worlds, and the pessimist knows it." (Robert Oppenheimer)
"You may be disappointed if you fail, but you are doomed if you don't try." (Beverly Sills)
However, do not use a comma before a coordinator that links two words or phrases: "Jack and Diane sang and danced all night."
2. Use a Comma to separate items in a series
Use a comma between words, phrases, or clauses that appear in a series of three or more:
"You get injected, inspected, detected, infected, neglected, and selected." (Arlo Guthrie)
"It is by the goodness of God that in our country we have three unspeakably precious things: freedom of speech, freedom of conscience, and the prudence never to practice either of them."
Notice that in each example a comma appears before but not after the coordinator.
3. Use a Comma after an introductory word group
Use a comma after a phrase or clause that precedes the subject of the sentence:
"When you get to the end of your rope, tie a knot and hang on."
"If at first you don't succeed, failure may be your style." (Quentin Crisp)
4. Use a Pair of Commas to set off interruptions
Use a pair of commas to set off (= resaltar, realzar) words, phrases, or clauses that interrupt a sentence:
"Words are, of course, the most powerful drug used by mankind."
"Literature is all, or mostly, about sex." (Anthony Burgess)
Semicolons, Colons, and Dashes
These three marks of punctuation can be effective when they are used correctly, and not too often.
Use a semicolon to separate two main clauses not joined by a coordinating conjunction:
Those who write clearly have readers; those who write obscurely have commentators.
We can also use a semicolon to separate main clauses joined by conjunctions like however, consequently, otherwise, moreover, nevertheless):
A great many people may think that they are thinking; however, most are merely rearranging their prejudices.
Basically, a semicolon (whether followed by a conjunction or not) serves to coordinate two main clauses.
Use a colon to set off a summary or a series after a complete main clause:
It is time for the baby's birthday party: a white cake, strawberry-marshmallow ice cream, and a bottle of champagne saved from another party. (Joan Didion)
Tip: To type a proper dash, press Alt + 0150 in your keyboard.
Use a dash to set off a short summary after a complete main clause:
At the bottom of Pandora's box lay the final gift– hope.
We may also use a pair of dashes in place of a pair of commas to set off words, phrases, or clauses that interrupt a sentence with additional –but not essential– information:
In the great empires of antiquity –
Unlike parentheses (which tend to de-emphasize the information contained between them), dashes are more emphatic than commas. And dashes are particularly useful for setting off items in a series that are already separated by commas.
PRACTICE: Creating sentences with Semicolons, Colons, and Dashes.
Use each sentence below as the model for a new sentence. Your new sentence should follow the accompanying rule and use the same punctuation contained in the model.
Levin wanted friendship and got friendliness; he wanted steak and they offered chopped pork. (Bernard Malamud, A New Life)
Rule: Use a semicolon to separate two main clauses not joined by a coordinating conjunction.
Your essay is both good and original; however, the part that is good is not original, and the part that is original is not good.
Rule: Use a semicolon to separate main clauses joined by a conjunction like however and nevertheless.
I divide all readers into two classes: those who read to remember and those who read to forget. (William Lyon Phelps)
Rule: Use a colon to set off a summary or a series after a complete main clause.
The senator reminded us that there is only one thing we can count on for sure –total uncertainty.
Rule: Use a dash to set off a short summary after a complete main clause.
Our labours in life –learning, earning, and yearning– are also our reasons for living.
Rule: For the sake of clarity or emphasis (or both), use a pair of dashes to set off words, phrases, or clauses that interrupt a sentence.
A sentence may end with a period (.), a question mark (?), or an exclamation mark (!).
Use a period at the end of a sentence that makes a statement. We find this principle at work in each of Iñigo Montoya's sentences in this speech from the movie The Princess Bride:
I was eleven years old. And when I was strong enough, I dedicated my life to the study of fencing. So the next time we meet, I will not fail. I will go up to the six-fingered man and say, "Hello. My name is Inigo Montoya. You killed my father. Prepare to die."
Notice that a period goes inside a closing quotation mark.
Use a question mark after direct questions, as in this exchange from the same movie:
The Grandson: Is this a kissing book?
Grandpa: Wait, just wait.
The Grandson: Well, when does it get good?
Grandpa: Keep your shirt on, and let me read.
However, at the end of indirect quotations (that is, reporting someone else's question in our own words), use a period instead of a question mark:
The boy asked if there was kissing in the book.
Now and again, we may use an exclamation mark at the end of a sentence to express strong emotion: Consider Vizzini's dying words in The Princess Bride:
You only think I guessed wrong! That's what's so funny! I switched glasses when your back was turned! Ha ha! You fool! You fell victim to one of the classic blunders! The most famous is never get involved in a land war in
Clearly (and comically), this is an extreme use of exclamations. In our own writing, we should be careful not to reduce the effect of the exclamation mark by overworking it (using it too much).
Adapted from: http://grammar.about.com/