The comma, semicolon, and colon are often misused because they all can indicate pauses.
Commas show a separation of ideas in a sentence.*
Commas go after introductory phrases.
Commas go after a transition word.
If two sentences are joined together without the comma and the coordinating conjunction (and, or, but, etc.) then it's a run-on sentence. For more information, visit the Run-On and Sentence Fragment page on the Grammar Guide.
Semicolons (;) are used to connect independent clauses (works best when the clauses are closely related).
Semicolons go before connecting words or phrases.
A colon (:) has three main uses:
After a word introducing a quote, explanation, example, or series.
Between independent clauses when the second explains the first, similar to a semicolon.
Quotations: Use a comma before a quotation when an introductory phrase is before the quote.
She said, "How are you today?"
Titles: If a title follows a name, separate the title from the rest of the sentence with a pair of commas.
Anna Johnson, MD, is a primary care doctor.
Addresses: Use a comma to separate each element in an address. Also, use a comma after a city-state combination within a sentence.
Numbers: In numbers more than four digits long, use commas to separate the numbers into groups of three, starting from the right. In numbers four digits long, a comma is optional.
3,500 (comma is optional)
Dates: In dates, the year is set off from the rest of the sentence with a pair of commas.
On December 7, 1941, Japan staged a surprise attack on Pearl Harbor.
Genesis 1:1 starts "In the beginning God created the heavens and the earth."
It's myth that the ratio of women to men in Nottingham is 6:1.
(Colons are used in timings greater than a minute.)