Use a colon:
To introduce a list of items. (This report is missing several sections: a table of contents, an introduction, and a conclusion.)
To introduce an explanation. (I’m not sure I agree with his conclusion: His logic seems flawed.)
To heighten the impact of the word or words that follow.
(The cause of her illness was simple: malnutrition.)
After the salutation of a business letter. (Dear Mr. Smith:)
To separate elements of time (hours and minutes, minutes and seconds). (The meeting starts at 3:30. Her official time for the race was 6:02:27.)
Do not place a colon between a verb and its objects or between a preposition and its objects.
Incorrect: Please address the letter to: the client, the CEO, and the lawyers. Correct: Please address the letter to the client, the CEO, and the lawyers.
Incorrect: Our strongest departments are: accounting, marketing, and human resources.
Correct: Our strongest departments are accounting, marketing, and human resources.
Place a comma:
Between two or more adjectives that modify a noun.
A dark, conservative suit is best for a job interview.
Between items in a series. The comma before the and or or in a series is now considered optional, but I include it to avoid confusion. Whether or not you choose to include this serial comma, be consistent throughout your document.
The flag is red, white, and blue.
To set off clauses that would not change the meaning of their subject if they were left out (“nonessential clauses”). If the words are essential to the meaning of the subject, do not enclose them in commas.
The conference room, which is comfortable but elegant, can hold 30 people.
Before a conjunction separating two independent clauses. However, if the clauses are extremely short, no comma is necessary.
She writes the documents within three to four weeks, and I edit them within a few days.
She writes and I edit.
To separate a direct quote from the rest of the sentence.
“This report is pathetic,” he yelled.
Between parts of dates and place names.
The agreement was signed on November 6, 2002, but didn’t take effect until a month later.
She will retire to Palm Beach, Florida, when she turns 70.
Between names and titles or degrees that follow.
Thomas Matthews, Jr.
Angela Adams, M.D.
Do not connect two individual sentences (known as independent clauses), each with its own subject and verb, by a comma. Instead, separate them with a semicolon or a period. If the clauses are short, you can also join them with a conjunction.
Incorrect: Yesterday’s meeting was there, today’s meeting is here.
Correct: Yesterday’s meeting was there; today’s meeting is here.
Correct: Yesterday’s meeting was there. Today’s meeting is here.
Correct: Yesterday’s meeting was there, while today’s meeting is here.
Modifiers are words or phrases that define or alter the meaning of something. Modifiers are dangling when they don’t modify the subject of the sentence.
Incorrect: Believing the product to be faulty, it was recalled. (Who believes the product to be faulty?)
Correct: Believing the product to be faulty, the company recalled it. (The company believes the product to be faulty.)