In a Time magazine essay titled "In Praise of the Humble Comma," Pico Iyer nicely illustrated some of the various uses of punctuation marks:
Punctuation, one is taught, 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.
Odds are that 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 (as we do in the Basic Grammar pages). Here, beginning with the end marks (periods, question marks, and exclamation points), we'll review the conventional uses of punctuation in American English.
A sentence may end with a period (.), a question mark (?), or an exclamation point (!).
Use a period at the end of a sentence that makes a statement. We find this principle at work in each of Inigo 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.
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 then we may use an exclamation point 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 Asia, but only slightly less well-known is this: never go in against a Sicilian when death is on the line! Ha ha ha ha ha ha ha! Ha ha ha ha ha ha ha!
Clearly (and comically), this is an extreme use of exclamations. In our own writing, we should be careful not to deaden the effect of the exclamation point by overworking it.