This article collects rules on how to place commas (and semi-colons) in English.
Between independent clauses
Independent clauses are always separated with a comma, when they are joined by for, and, nor, but, or, yet, so (in the sense of as a result):
The night was cold, and the sky was clear.
Two clauses are independent, if each of them can (syntactically) stand alone.
Dependent and independent sentences
When an independent and a dependent/subordinate phrase appear in a sentence, a comma is used if the dependent sentence precedes the independent sentence. A subordinate clause is generally indicated by a subordinating conjunction such as after, although, as, as soon as, because, before, by the time, even if, even though, every time, if, in case, now that, once only if, since, though, unless, until, when, whenever, whereas, while.
(1) [dependent before independent clause] If I pass the test, (then) I will give a party.
(2) [dependent after independent clasue] I will give a party if I pass the test.
(3) [unnecessary word/particle] Well, then I should go.
Connectives in a sentence-initial position are always followed by a comma.Some examples of connectives are as a result, consequently, furthermore, hence, however, moreover, so, therefore, thus.
(1) The sun goes down. Therefore, we should go back home.
(2) The tree was diseased. So, he was not strong enough to withstand last week’s storm.
Comma as separator in lists
Clearly, the comma should be used to separate items in a list. Whether to place a comma before the final and or or is a matter of choice. If the list items are longer and contain commas, a semicolon may also be used as list separator.
(1) [Comma] They bought bread, two bottles of milk(,) and some cheese.
(2) [Semi-colon] Three questions arise: whether to leave the house, even if it is raining; whether to go shopping at the mall or at the small shop around the corner, if at all; and whether to take along an umbrella.
Direct speech and and inquit (“he say”, “she claims”,…) are separated with a common, when they appear in one sentence. If the inquit follows the direct speech part, the comma appears before the closing quotes; if the inquit precedes the direct speech part, the comma appears in front of the closing quotes.
(1) [inquit before quotation] Erich Honecker said, “Nobody intends to build a wall.”
(2) [inquit after quotation] “Nobody intends to build a wall,” said Erich Honecker.
(3) [inquit between quotations] “Nobody,” said Erich Honecker, “intends to build a wall.”
(4) [inquit, quotation, and other phrase] Erich Honecker, “Nobody intends to build a wall,” and smiled.
Separate repeated words
If the same word (or number) is immediately repeated, a comma should separate them to avoid confusion:
(1) Whatever you say, say it clearly.
(2) In 1998, 101 people died in the train disaster near Eschede.
Commas in dates
When listing two parts of a date (e.g., December 30, August 1980, June 1), do not use a comma to separate them. When listing more than two parts of a date, use a comma:
(1) [no comma] On January 6, three children rang at my door.
(2) [use comma] The bombardment of Baghdad started on March 20, 2003.
Commas and numbers
A comma may be used as thousands separator (e.g., in 5,172,455), but not in four-digit years (cf. 1997 vs. 10,000 B.C.), zip codes (e.g., Beverly Hills 90210), or page numbers.
A comma may split related units (also in an abstract sense)
(1) [feet and inches] I am five feet, three inches high.
(2) [act and scene] The most exciting part was was act 2, scene 3.
(3) [page and line] See also page 179, line 12.
Commas and names, places, etc.
Names are separated from academic degree by a comma:
I would now like to welcome Mr. Turner, PhD.
Similarly, cities are separated from counties/states/… by comma:
I come from Stuttgart, Baden-Württemberg, Germany.
Use a comma after the greeting in an informal letter, use a colon for formal letters.
(1) [informal] Dear Dave,
(2) [formal] Dear Mr. Burton:
Use a comma after the closing words:
(1) [informal] Yours, /Love,
(2) [formal] Sincerely, /Regards,
-  The 5 Uses of a Comma in English:
-  MyEnglishTeacher.net
-  Commas in English – Ego4U.com