Commas are probably the most frequently used piece of punctuation, so it is not surprising that comma errors often cause writers more problems than any other type of punctuation errors. But the solution to these problems is not difficult as most comma errors can be eliminated if writers will learn just seven rules:
Use commas to separate items in a series.
Example: I bought milk, eggs, and cheese.
Comment: A series must contain at least three items. Some people have been taught that the comma before the “and“ is optional, but when in doubt about what your audience finds acceptable, you should use the comma before “and.”
Use commas to set off introductory material.
Example: After I went to the store, I went to visit Mrs. Smith.
Comment: One way to identify introductory material is to find the subject of the sentence and then note whether or not there is anything to the LEFT of the subject. If so, place a comma at the end of the material, just before the subject. “Subject” in this case refers to the grammatical subject as well as any words connected to it (articles and/or modifiers).
Use commas on both sides of words that interrupt the flow of thought in a sentence.
Example: After I went to the store, I went, even though I didn’t really want to, to visit Mrs. Smith.
Comment: Interrupters can be single words as well as multiple word units. If you could delete the word(s) without changing the meaning of the sentence, then you can apply this rule.
Use commas to join two complete sentences connected by “for,” “and,” “nor,” “but,” “or,” “yet,” “so” (the coordinating conjunctions or FANBOYS).
Example: I ran errands this morning, but I would have rather spent the time reading a book.
Comment: When “so” means “so that,” it is not a coordinating conjunction, nor is “yet” a coordinating conjunction when it refers to time. In both of these situations, this comma rule does NOT apply.
Use commas to set off a direct quotation from the rest of the sentence.
Example: When I ran into Mrs. Smith at the grocery store, she said, “Why don’t you come over for awhile when you get finished?”
Comment: This rule ONLY applies when the quote is preceded by some type of speaker tag (e.g. she writes, she said, according to John Smith, Smith argues, etc.).
Use commas to set off unnecessary, additional information at the end of a sentence.
Example: After I went to the store, I went to visit Mrs. Smith, dragging my feet all the way.
Comment. Before you apply this rule, be sure that the information you plan to set off is unnecessary to the meaning of the sentence. Be especially cautious when the material at the end of the sentence begins with a subordinating conjunction (e.g., because, even though, if, while, etc. — consult a handbook for a complete list); only rarely will such material need to be preceded by a comma.
Use commas in certain everyday situations.
Example: I had last seen Mrs. Smith on June 12, 1975, in Seattle, Washington.
Comment: This rule covers a number of common uses of the comma: between cities and states, in numbers (e.g., 10,000), between a date and year, and after a salutation or closing in a letter.
(Except for the sixth rule, these rules are from John Langan’s College Writing Skills).