1Ø1     Separate the introductory phrase from the rest of a sentence 

After taking a hot shower, Eugene fell asleep on the sofa.

1Ø2     Set apart words that interrupt the flow of the sentence

                                    Hans, who was not properly dressed, said he would be at the door in a minute.

1Ø3     Separate two complete thoughts connected by a “joining” word (but, so, and) 
         I wanted to go outside, but it was raining cats and dogs!
Only use a comma with these joining words if they join two complete thoughts.   
When one clause is dependent there is no comma before the conjunction.
    It was raining cats and dogs because of the alien invaders.
1Ø4     Separate a direct quotation from the rest of a sentence

He shouted, “Look out below!”

1Ø5     Be used between items in a series

The comma is used to list three or more items in a series. 
The choices of color were red, green, yellow, and blue.
(The comma before the "and" is optional, but you must
    either use it always or never use it.)
You can Return to www.WritingResource.org/  or below you can find
       reiterations of all of the various rules about commas below!

Comma rules  
Guidelines and examples for comma use: from St. Cloud State University.

Rules for comma usage
Eleven tips, with examples and links to several quizzes, from the Guide to Grammar and Writing.

Proofreading for commas
Guidelines on six main points for comma use: from the Purdue Online Writing Lab.

Using commas
Guidelines on six uses of commas, with examples, from the University of Richmond.

The comma
Do's and don'ts for comma use, with a review quiz, from the University of Ottawa.

Recognizing comma problems
Brief discussion of avoiding comma errors: from the University of Victoria.

Commas with coordinating conjunctions

Commas with introductory elements

Commas with quotations

Unnecessary commas