"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.

Independent Clauses

Rule #1: Comma Needed

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.

• They are just finishing up the report, so they are almost ready to present it to the boss. (same subject)
• She didn't like the restaurant I recommended, but her husband really enjoyed it. (different subjects)
• Discuss these questions with your group, and be sure to take notes during the discussion. (imperative verbs)

Rule #2: No Comma Needed

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).

• We finished the report and presented it to the boss.
• They want to go skiing and snowboarding over the holidays.
• I'll call him tonight or email him tomorrow.

Rule #3: Comma Optional

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.

• I cooked and she cleaned.
• Lisa made cookies and Pedro baked a cake.
• Raise your right hand and repeat after me.

Dependent Clauses

Rule #4: Comma Needed

If the dependent clause comes before the independent clause, use a comma.

• Even though it was raining, I didn't bring an umbrella.
• Because you reminded him, he brought the necessary documents to the meeting.
• If she calls in sick, I will take over for her.

Rule #5: No Comma Needed

If the dependent clause comes after the independent clause, don't use a comma.

• I didn't bring an umbrella even though it was raining.
• He brought the necessary documents to the meeting because you reminded him.
• I will take over for her if she calls in sick.

Conclusion

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