Commas are not always a matter of life and death, but using them judiciously and correctly certainly reduces ambiguity.
Follow these tips to avoid about 90% of comma errors.
- Use a comma to separate two independent clauses joined by a coordinating conjunction (and, but, or, yet, for, nor, so).
- Otherwise, do NOT use a comma to splice together two independent clauses. See my page on comma splices.
- Use commas to separate the items in a list.
- A comma is required after the penultimate (second-last) item in the list: “God, Adam, and Eve took their evening constitutional in the garden.” This comma is sometimes called the Oxford comma or the serial comma. See OC.
- Use a pair of commas (or dashes or parentheses) to separate a subordinate clause or parenthetical expression from the main clause.
See also Barney Latimer’s post at The MLA Style Center: “Commas: Essential and Nonessential Elements.”