How to avoid a common comma mistake
It’s amazing how much trouble a tiny little punctuation mark can cause. And no other mark is used—or misused—as often as the comma.
One of the most common uses of the comma is to join independent clauses. The rule for using a comma before a conjunction (and, but, or, nor, for, so, yet) between two independent clauses is actually quite simple, but it is one of the errors editors find themselves correcting most often.
What is an independent clause? A group of words that can stand alone as a sentence and contains both a subject and a verb. So if two independent clauses are joined by a conjunction, you’ll need to add a comma before the conjunction.
Example: The weather was bitterly cold, so I decided never to leave my house again.
Here we have two independent clauses joined by the conjunction so. It’s pretty easy to tell they are independent, because both of them work on their own as sentences. Plus, they both have a subject and a verb (weather, was; I, decided).
A common mistake is to add a comma before a conjunction like and or but even when it is not being used to separate two independent clauses.
Take this example: I decided to never leave my house again and then proceeded to stockpile canned goods and bottles of water.
This is called a compound predicate, which is really one independent clause with one subject that governs two verbs. No need for the comma before and here; the second half of the sentence contains a verb, proceeded, but it is still governed by the subject I at the beginning of the sentence.
Still pretty easy. But there are cases that aren’t so easy.
One last example: I ventured out of my self-imposed hibernation when I went to work on Monday, and on Tuesday decided that it was finally warm enough to rejoin society for good.
Some editors may remove the comma before “and on Tuesday,” since the subject I governs the verb decided. However, The Copyeditor’s Handbook by Amy Einsohn dictates an exception in cases like these, when the comma will prevent a misreading of “I went to work on Monday and on Tuesday” as a unit of thought.
Isn’t it nice that the rules of punctuation can be flexible sometimes?