Commas can be a bit tricky, I’ll admit. When I teach class, students usually have a lot of question about them—so many that I finally wrote a book about commas. And when, after editing, I’m proofreading a client’s document or manuscript, I spend a lot of time fixing commas. Sometime I add commas, and sometimes I take them out.
Like I said, they’re tricky. The two samples below represent common confusion, and common mistakes, with commas. I’ll give you a hint: they are both correct as is. The question, therefore, is why don’t we need to add any commas?
We discovered that the ion plays a role in fuel production [comma?] and the ion is not detectable using X-ray technologies.
We know the rule about putting a comma before a coordinating conjunction that joins two independent clauses (Rule D), as in “The dog ran, and the cat slept.” This rule seems to apply to the first example. “We discovered . . . fuel production” looks like one independent clause. “The ion is not . . . technologies” looks like another. Shouldn’t a comma be before “and”?
No, and here’s why. The subject of the sentence is “We,” and the main verb is “discovered.” The direct object of “discovered” is “that the ion plays a role in fuel production.” Now, let’s look more carefully. We realize that, in fact, two things were discovered:
1. that the ion plays a role in fuel production, and
2. that the ion is not detectable using X-ray technologies.
That entire second part is NOT a new independent clause. Instead, it is a direct object of “discovered,” which has the subject “We.” This sentence has only one subject and verb: “We discovered.” With only one subject and verb, we have only one independent clause, not two!
You might also see that the second part has an implied “that,” as in “that the ion is not….” When we put “that” back in the sentence, we see, again, that the second part is not an independent clause. As a result, Rule D doesn’t apply. Instead, we apply Rule E: Use commas [or not] as if implied words were present.
Following the seminar, participants were more comfortable with public speaking [comma?] because they had ample practice opportunities.
This one also looks like two independent clauses joined by a conjunction. Here, the conjunction is “because.” The second part, reading “they had ample practice opportunities,” looks like an independent clause, but it’s not.
If we put “and” between two independent clauses, the “and” only serves as a joining word for two separate thoughts. In this case, however, “because” is an essential part of the second idea. It doesn’t join a new idea to the first one. Rather, it is part of the new idea. It has meaning in this sentence, indicating causality. Rather than signaling to the reader that a new independent idea is about to begin, it is the first word of a dependent clause.
Here’s another way to look at this. Whereas “and” and “but” are coordinating conjunctions, “because is a subordinating conjunction. In short, Rule D doesn’t apply, and we don’t need a comma before “because.”
This gives us Rule F: Don’t use a comma before “because” when joining two independent clauses (which it isn’t actually doing
What have we learned about commas? Commas are placed for very specific reasons, and only for those reasons. If we don’t have a reason to use a comma, we don’t. If we do, we do.
Need help with commas? Get Zen Comma, an instructive reference guide on the 17 major uses and misuses of commas, available in PDF and Kindle formats. Read more about Zen Comma.