When should I capitalize?

Use a period at the end of a command.

Hand in the poster essays no later than noon on Friday.
In case of tremors, leave the building immediately.

Use a period at the end of an indirect question.

The teacher asked why Maria had left out the easy exercises.
My father used to wonder why Egbert's ears were so big.
Use a period with abbreviations:

Dr. Espinoza arrived from Washington, D.C., at 6 p.m.
Notice that when the period ending the abbreviation comes at the end of a sentence, it will also suffice to end the sentence. On the other hand, when an abbreviation ends a question or exclamation, it is appropriate to add a question mark or exclamation mark after the abbreviation-ending period:

Did you enjoy living in Washington, D.C.?

Occasionally, a statement will end with a question. When that happens, it is appropriate to end the sentence with a question mark.

We can get to Boston quicker, can't we, if we take the interstate?
His question was, can we end this statement with a question mark?
She ended her remarks with a resounding why not?

The comma is a valuable, useful punctuation device because it separates the structural elements of sentences into manageable segments. The rules provided here are those found in traditional handbooks; however, in certain rhetorical contexts and for specific purposes, these rules may be broken.

1. Use commas to separate independent clauses when they are joined by any of these seven coordinating conjunctions: and, but, for, or, nor, so, yet.

The game was over, but the crowd refused to leave.
The student explained her question, yet the instructor still didn't seem to understand.
Yesterday was her brother's birthday, so she took him out to dinner.
2. Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.

a. Common starter words for introductory clauses that should be followed by a comma include after, although, as, because, if, since, when, while.
While I was eating, the cat scratched at the door.
Because her alarm clock was broken, she was late for class.
If you are ill, you ought to see a doctor.
When the snow stops falling, we'll shovel the driveway.

However, don't put a comma after the main clause when a dependent (subordinate) clause follows it (except for cases of extreme contrast).

1. She was late for class, because her alarm clock was broken. (incorrect)
2. The cat scratched at the door, while I was eating. (incorrect)
3. She was still quite upset, although she had won the Oscar. (correct: extreme contrast)

b. Common introductory phrases that should be followed by a comma include participial and infinitive phrases, absolute phrases, nonessential appositive phrases, and long prepositional phrases (over four words).

Having finished the test, he left the room.
To get a seat, you'd better come early.
After the test but before lunch, I went jogging.
The sun radiating intense heat, we sought shelter in the cafe.

c. Common introductory words that should be followed by a comma include yes, however, well.

Well, perhaps he meant no harm.
Yes, the package should arrive tomorrow morning.
However, you may not be satisfied with the results.

3. Use a pair of commas in the middle of a sentence to set off clauses, phrases, and words that are not essential to the meaning of the sentence. Use one comma before to indicate the beginning of the pause and one at the end to indicate the end of the pause.

Here are some clues to help you decide whether the sentence element is essential:

If you leave out the clause, phrase, or word, does the sentence still make sense?
Does the clause, phrase, or word interrupt the flow of words in the original sentence?
If you move the element to a different position in the sentence, does the sentence still make sense?
If you answer "yes" to one or more of these questions, then the element in question is nonessential and should be set off with commas. Here are some example sentences with nonessential elements:

Clause: That Tuesday, which happens to be my birthday, is the only day when I am available to meet.
Phrase: This restaurant has an exciting atmosphere. The food, on the other hand, is rather bland.
Word: I appreciate your hard work. In this case, however, you seem to have over-exerted yourself.
4. Do not use commas to set off essential elements of the sentence, such as clauses beginning with that (relative clauses). That clauses after nouns are always essential. That clauses following a verb expressing mental action are always essential.

That clauses after nouns:

The book that I borrowed from you is excellent.
The apples that fell out of the basket are bruised.
That clauses following a verb expressing mental action:

She believes that she will be able to earn an A.
He is dreaming that he can fly.
I contend that it was wrong to mislead her.
They wished that warm weather would finally arrive.
Examples of other essential elements (no commas):

Students who cheat only harm themselves.
The baby wearing a yellow jumpsuit is my niece.
The candidate who had the least money lost the election.
Examples of nonessential elements (set off by commas):

Fred, who often cheats, is just harming himself.
My niece, wearing a yellow jumpsuit, is playing in the living room.
The Green party candidate, who had the least money, lost the election.
Apples, which are my favorite fruit, are the main ingredient in this recipe.
Professor Benson, grinning from ear to ear, announced that the exam would be tomorrow.
Tom, the captain of the team, was injured in the game.
It is up to you, Jane, to finish.
She was, however, too tired to make the trip.
Two hundred dollars, I think, is sufficient.
5. Use commas to separate three or more words, phrases, or clauses written in a series.

The Constitution establishes the legislative, executive, and judicial branches of government.
The candidate promised to lower taxes, protect the environment, reduce crime, and end unemployment.
The prosecutor argued that the defendant, who was at the scene of the crime, who had a strong revenge motive, and who had access to the murder weapon, was guilty of homicide.
6. Use commas to separate two or more coordinate adjectives that describe the same noun. Be sure never to add an extra comma between the final adjective and the noun itself or to use commas with non-coordinate adjectives.

Coordinate adjectives are adjectives with equal ("co"-ordinate) status in describing the noun; neither adjective is subordinate to the other. You can decide if two adjectives in a row are coordinate by asking the following questions:

Does the sentence make sense if the adjectives are written in reverse order?
Does the sentence make sense if the adjectives are written with and between them?
If you answer yes to these questions, then the adjectives are coordinate and should be separated by a comma. Here are some examples of coordinate and non-coordinate adjectives:

He was a difficult, stubborn child. (coordinate)
They lived in a white frame house. (non-coordinate)
She often wore a gray wool shawl. (non-coordinate)
Your cousin has an easy, happy smile. (coordinate)
The 1) relentless, 2) powerful 3) summer sun beat down on them. (1-2 are coordinate; 2-3 are non-coordinate.)
The 1) relentless, 2) powerful, 3) oppressive sun beat down on them. (Both 1-2 and 2-3 are coordinate.)
7. Use a comma near the end of a sentence to separate contrasted coordinate elements or to indicate a distinct pause or shift.

He was merely ignorant, not stupid.
The chimpanzee seemed reflective, almost human.
You're one of the senator's close friends, aren't you?
The speaker seemed innocent, even gullible.
8. Use commas to set off phrases at the end of the sentence that refer back to the beginning or middle of the sentence. Such phrases are free modifiers that can be placed anywhere in the sentence without causing confusion. (If the placement of the modifier causes confusion, then it is not "free" and must remain "bound" to the word it modifies.)

1. Nancy waved enthusiastically at the docking ship, laughing joyously. (correct)
2a. Lisa waved at Nancy, laughing joyously. (incorrect: Who is laughing, Lisa or Nancy?)
2b. Laughing joyously, Lisa waved at Nancy. (correct)
2c. Lisa waved at Nancy, who was laughing joyously. (correct)
9. Use commas to set off all geographical names, items in dates (except the month and day), addresses (except the street number and name), and titles in names.

Birmingham, Alabama, gets its name from Birmingham, England.
July 22, 1959, was a momentous day in his life.
Who lives at 1600 Pennsylvania Avenue, Washington, DC?
Rachel B. Lake, MD, will be the principal speaker.
(When you use just the month and the year, no comma is necessary after the month or year: "The average temperatures for July 1998 are the highest on record for that month.")

10. Use a comma to shift between the main discourse and a quotation.

John said without emotion, "I'll see you tomorrow."
"I was able," she answered, "to complete the assignment."
In 1848, Marx wrote, "Workers of the world, unite!"
11. Use commas wherever necessary to prevent possible confusion or misreading.

To George, Harrison had been a sort of idol.
Comma Abuse