Why Is Comma Usage so Confusing? Tackling Commas and Conjunctions

Laura's Lessons
Choosing when to use a comma can be so frustrating! Some people use too many, some people don’t use enough, and pretty much everyone can agree that there are too many “rules” for writers to keep straight. Plus, those rules can differ based on each writer’s (and editor’s) style and preference…hence the confusion. Let’s gently dip our toes into comma usage and tackle how to use commas with conjunctions.

Remember that Schoolhouse Rock song? Conjunctions link parts of sentences together. You use them all the time—some common conjunctions are “and,” “but,” “or,” and “so.” Writers often like to throw a comma before every single conjunction, but this isn’t always necessary (or correct).

The easiest rule to follow here is to only put a comma before a conjunction if the conjunction links two complete sentences (ones that stand on their own with both a subject and a verb). But even here it’s not always necessary. I promise that’s not as confusing as it sounds!


Here’s an example:

I initially wanted to order a pizza for my PANCE review session but chose Thai food instead.

Let’s look at the parts of this sentence linked with the conjunction “but”: “I initially wanted to order pizza for my PANCE review session” is a full sentence—it can stand on its own. “Chose Thai food instead” is not a full sentence because there’s no subject. We don’t know who or what chose Thai food. Therefore, a comma doesn’t belong before “but.”

I got to my OB/GYN shelf exam an hour early so I grabbed a snack.

“I got to my OB/GYN shelf exam an hour early” is a full sentence. “I grabbed a snack” is also a full sentence. Here, we’ve linked these two full sentences with a conjunction, so we could add a comma before “so” (I got to my OB/GYN shelf exam an hour early, so I grabbed a snack.) However, this comma is not completely necessary. Shorter sentences like this are still clear without the comma, so adding a comma is up to the writer or editor’s preference.


Let’s switch gears to some medical-based examples:

A 23-year-old G1P0 woman presents at 18 weeks gestation and no fetal heart motion is seen on ultrasound.

Here we have two complete sentences: A 23-year-old G1P0 woman presents at 18 weeks gestation; No fetal heart motion is seen on ultrasound. Therefore, we can add that comma before “and.” (A 23-year-old G1P0 woman presents at 18 weeks gestation, and no fetal heart motion is seen on ultrasound.) Unlike the previous example, the comma is helpful here—it adds a clarifying pause in the sentence so we don’t think “and” will lead to more information about the woman.

An echogenic intracardiac focus is found in 3–5% of normal fetuses but is also found in approximately 25% of fetuses with trisomy 21.

Do you think a comma should come before “but”? Nope! “Is also found in approximately 25% of fetuses with trisomy 21” isn’t a complete sentence—we don’t know what is also found in approximately 25% of fetuses with trisomy 21. No comma!


Avoiding unnecessary commas is a good thing. However, if the comma before a conjunction (that links two complete sentences, of course) will add a clarifying pause and avoid confusing the reader, it’s a helpful little tool.

Check out Laura’s most recent lessons:

How to Choose Between I.E. and E.G.
How to Easily Remember Affect vs Effect
When Not to Use a Colon


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