Few indeed are the writers who fully understand the use of commas. Part of the problem is inherent: a wide range of comma usage is, in my opinion, discretionary. It’s not a matter of knowing the rules (hence the uselessness of relying on half-remembered “rules of thumb”), but of being conscious of the range of uses for this most subtle of punctuation marks and being able to explain the reasoning behind a particular usage.
In my opinion, there are a handful of situations where a comma is more or less obligatory. The first is in the construction of a series: “red white and blue” is clearly wrong. There is some dispute over whether it should be “red, white, and blue” (where the comma before the final conjunction is known as the “Oxford comma”) or “red, white and blue” (as it would appear in many journalistic style manuals). I prefer the Oxford comma because I believe that omitting it implies too strong a relationship between the final two items in the series; others believe that the final “and” is already doing the work of separating the items in the series and the comma is superfluous.
If the style manual for the publication you are writing for (or the person grading your paper) specifies one way or the other, you should of course obey. If you are writing for your own edification, you should meditate on this question, come to a firm position one way or the other, and stick with it. Consistency is the important thing in this case, because it just does not seem to me that there could be a plausible stylistic reason for alternating between the two (except perhaps if one were writing a narrative piece about people who disagreed on this usage and therefore decided to reflect each character’s preference in the dialogue).
The second nearly obligatory situation is in a compound sentence — i.e., a sentence in which two independent clauses are joined by a conjunction. In these cases, a comma should almost always precede the coordinating conjunction. For instance, the sentence “Jack went home and Jill stayed at the bar” is not using the normative punctuation, because both the units of the sentence have a subject and predicate of their own. The prefered punctuation in most cases is instead “Jack went home, and Jill stayed at the bar.” Even here, however, there is some stylistic wiggle room, because omitting the comma could imply a tighter correlation between the two clauses — but the comma should be the rule, whereas any exception should be conscious and reasoned. However, in a sentence with either a compound subject or compound verb, a comma would be incorrect: “Jack, and Jill went to the bar” and “Jack sat down, and ordered a drink” are both wrong and should be written without a comma. Unless you’re E.E. Cummings, I don’t think there is wiggle room here.
The really tricky part, however, is that if the compound sentence is a subordinate clause, the comma should not be used because it would give a misleading impression of the grammatical structure of the sentence. So one would write “Jack was cleaning his apartment, and Jill had to wash her hair,” but by contrast, one would write “John had to go out alone because Jack was cleaning his apartment and Jill had to wash her hair.” Introducing a comma into the subordinate clause would imply that “John had to go out alone” and “Jill had to wash her hair” were at the same grammatical “level” within the sentence, which is misleading.
Where it starts to become fuzzy is the use of commas to offset interjections and particles. It seems to me that “Clearly, he was drunk” and “Clearly he was drunk” are both correct and the usage depends on stylistic preference. Omitting the comma seems to me to be more emphatic (cf. “Of course, you have to use commas correctly” and “Of course you have to use commas correctly”), while using the comma implies a more leisurely pace and a looser connection between the particle and the sentence. The latter usage could be thought of as a kind of “soft” parentheses.
The same discretion applies to subordinate clauses introduced by conjunctive adverbs such as “because” or “although.” Neither “he pulled over because he had to pee” nor “he pulled over, because he had to pee” is incorrect in itself, though one might be preferable in a given context; the choice is a stylistic one. The one tip that I would give you in this connection is not to pile them on in such situations. For instance, if the second independent clause of a compound sentence is introduced by an interjection or particle, do not introduce a comma after the conjunction to set it off. Here’s an example:
- Incorrect: Jack was unable to hang out that night, and, because Jill had to wash her hair, John had to go out by himself.
- Correct: Jack was unable to hang out that night, and because Jill had to wash her hair, John had to go out by himself.
The second comma in the incorrect example adds no greater clarity — it is simply redundant. One should think of the coordinating conjunction as “swallowing” the comma that would come before the interjectory phrase. Using a redundant comma in this situation bespeaks an insecure approach to punctuation, where one is willing to sacrifice elegance out of fear of omitting a comma anywhere it might plausibly go. (The same applies when the interjectory phrase is preceded by a subordinating pronoun. As an exercise, find an example of this usage in the present blog post.)
Most quotations do not need to be introduced with a comma. In my opinion, introducing dialogue with a comma should be reserved solely for dialogue in narrative pieces of writing. In all other situations, the quotation should be integrated into your own syntax. Examples:
- Incorrect: Christians believe that, “God created the heavens and the earth.”
- Correct: Christians believe that “God created the heavens and the earth.”
As should be clear from this brief discussion, the proper use of commas depends on two crucial factors: understanding the grammatical structure of your sentence and being self-aware about your stylistic preferences (both globally and in particular situations). If you can talk about why you used a comma in a certain way — other than “but I thought that was the rule” — then you are well on your way to being a comma professional like me.