When to Use a Comma Before “So”

Ah! We wish a hard-and-fast rule existed—such as, “Always (or never) use a comma with ‘so’ in a compound sentence.” Instead, you’ll need to do a lot of investigative work. You must determine the function of the word “so” in order to know whether to use a comma. Are you using “so” as a coordinating conjunction? When the answer is “yes,” place a comma before the word. Are you using “so” as a subordinating conjunction? If that’s the case, omit the comma.
To make matters more complicated, sometimes it’s appropriate to place a comma after the word “so.” In this article, we’ll give examples of all of the comma rules for the word “so,” along with a few other handy shortcuts.

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The Quick Test

The fastest way to tell whether “so” is behaving as a coordinating conjunction or a subordinating conjunction is to replace the word with similar parts of speech. 

Coordinating Conjunction Substitution

Remember FANBOYS, the mnemonic that lists the coordinating conjunctions?

F – For

A – And

N – Nor

B – But

O -Or

Y – Yet

​S – So

In the following example, you can see that “so” acts as a coordinating conjunction when you are able to replace it with “and so” without changing the meaning of the sentence. 

I grew up in Connecticut, so I frequently visited New York City on school field trips.

I grew up in Connecticut, and so I frequently visited New York City on school field trips.

Because you can use the two coordinating conjunctions, “and so,” to replace “so,” it’s safe to assume that “so” acts as a coordinating conjunction in this instance. You can feel confident placing a comma before “so” in this example. 

Subordinating Conjunction Substitution

Let’s look at another example where “and so” doesn’t work nearly so well:

I grew up in Connecticut so my mom could commute into the city. 

I grew up in Connecticut, and so my mom could commute into the city. 

Adding “and so” changes the meaning of the sentence above. The original sentence makes clear that the speaker lived in Connecticut because of the easy commute. In contrast, the second sentence could be interpreted differently. It almost sounds as if the speaker “grew up” so that the commute could take place. 

In this case, it would be more appropriate to replace the word “so” with “so that.”

I grew up in Connecticut so that my mom could commute into the city.

Because you can replace “so” with “so that,” you know you’re dealing with a subordinate clause. When joining a subordinate and a main clause, you won’t need a comma. 

What Makes the Substitutions Work?

In all of the compound sentences above, you can identify two clauses. In the first example, the content within each clause is independent. 

I grew up in Connecticut. I visited New York City on school field trips.

In the second sentence, there’s a main clause and a subordinate clause. Even though both clauses could stand alone as complete sentences, it’s obvious that the second clause depends on the introductory phrase to provide context. 

I grew up in Connecticut. My mom could commute to the city. 

Although “my mom could commute to the city” might form a grammatically complete sentence, it’s not a complete thought. Within the compound sentence, the second clause depends on the first. Because “so” serves as a subordinating conjunction here, you won’t need a comma.

Using Commas After So

In casual writing and speech, people sometimes start a sentence with a coordinating conjunction. Although many grammar-lovers hate it, plenty of sentences begin with “and,” “so,” and “but.” In fact, the Chicago Manual of Style asserts that, as long as the resulting sentence is perfectly clear, starting a sentence with a conjunction is grammatically correct. When a sentence begins with “so” (especially an informal sentence), it’s appropriate to include a comma. 

So, what do you think?

Similarly, when “so” completes a quotation, the word is often followed by a comma. 

Examples: 

“She couldn’t believe it was so,” he admitted.

“So,” she asked, “How many people are you expecting?” 

Rules to Remember

When you’re writing the word “so,” you’ll need to ask yourself several questions before you punctuate.

  1. Is “so” the first word in a sentence? If it is, you may want to follow it with a comma. 
  2. Could I replace “so” with “and so”? If yes, you should include a comma before the word “so.”
  3. Does it make more sense to replace “so” with “so that”? If yes, omit the comma.
  4. Does the word “so” appear within a quotation? If yes, remember to punctuate the quotation according to the standard rules. In some cases, that may involve a comma after the word “so.”

Sources:

  1. https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Usage/faq0013.html
  2. https://www.dailywritingtips.com/punctuating-so-at-the-beginning-of-a-sentence/
  3. https://dictionary.cambridge.org/us/grammar/british-grammar/conjunctions