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"Effective use of the comma involves good judgment, with ease of reading the end in view."
—The Chicago Manual of Style
How can such a little punctuation mark cause so much trouble for both English language learners and native speakers? There are so many cases where commas are needed (such as with non-restrictive adjective clauses or multiple adjectives), where it's your choice to use them or not (such as with the serial comma), and where they shouldn't be used (such as with restrictive adjective clauses). Also, as more of us text and write on our computers or other mobile devices, comma usage seems to be dwindling.
So where does this leave students who are trying to learn the correct way to use commas? Luckily, there are a few basic rules that will still be considered correct for a long time to come.
If two independent clauses are joined by a conjunction (and, but, so, or), use a comma. This is true whether the subject is the same or different in both clauses. This is also true for imperative sentences where the subject is dropped.
If the conjunction is only joining parts of the independent clauses (i.e., the second subject and/or verb is dropped because it is the same), no comma should be used. However, if you teach higher-level students, you may want to point out that a comma could be added if the first clause is quite long (for the sake of readability—see the conclusion below).
If the two independent clauses are very short, you can omit the comma. As noted in the intro, the trend nowadays is to omit the comma wherever possible. Note that students might be wondering how they can tell if a clause is 'short.' Remind them that if they're in doubt, use a comma because that will always be considered correct. In fact, you may not want to mention this rule to your lower-level students—it might cause unnecessary confusion.
If the dependent clause comes before the independent clause, use a comma.
If the dependent clause comes after the independent clause, don't use a comma.
Tell your students they can feel confident that their written work will always be correct if they follow the rules above. But make sure they also understand that some commas are optional (or can be added or deleted for readability even when it's not the usual rule). They will come across instances where the rules aren't followed to a T (and that's usually okay). The good news is that even if they make a mistake with the rules above, it won't always be considered wrong. If there is another comma case you'd like me to blog about, just leave a comment below!
Source: The Chicago Manual of Style, 16th edition, sections 6.16–6.32
