Do You Use a Comma Before “So”?

Should you place a comma before so when it joins two clauses in a sentence? The answer depends on whether the clause introduced by so is an independent or dependent clause. If so begins an independent clause, a comma should precede it, but if it begins a dependent clause, leave it out.

Let’s have a look at how commas are used before so in the middle of a sentence.

Use a Comma with “So” + an Independent Clause

An independent clause is a clause that would convey a complete thought if it were to be set apart as a sentence on its own. In literary terms, it is a clause that can stand on its own two feet. Here is an example of a sentence consisting of two independent clauses.

The eruption of Mount Vesuvius destroyed a great city, but some of Pompeii’s frescoes were preserved in the ash.

There are two independent clauses here, joined by the coordinating conjunction but. Although it would result in a more stilted writing style, each could stand separately as a sentence and still be correct.

The eruption of Mount Vesuvius destroyed a great city.

Some of Pompeii’s frescoes were preserved in the ash.

So is one of seven coordinating conjunctions represented by the mnemonic FANBOYS: for, and, nor, but, or, yet,and so. When these coordinating conjunctions connect two independent clauses, the conjunction is always preceded by a comma.

The grocery store was out of tomatoes, so I borrowed some from my neighbor.

Daniel had the highest score in math in the whole school, so he was made principal for the day.

Simple, right? Not exactly, because one of the seven FANBOYS conjunctions listed above is leading a double life—and it happens to be so.

Don’t Use a Comma with “So” + a Dependent Clause

So can also be used as a subordinating conjunction to connect an independent clause and a dependent clause. A dependent clause needs an independent clause to form a complete thought.

I ran for shelter when it began to rain.

In this example, when it began to rain could not stand on its own as a complete thought. It leaves the reader asking what happened when it began to rain. After all, it begins with the subordinating conjunction when.

So can also be used as a subordinating conjunction, and when it is used this way, it is not preceded by a comma.

I went to the store so I could buy tomatoes.

Carl studied hard so he could pass the test.

A Quick Trick for Deciding If You Need a Comma before “So”

If you are unsure if you should place a comma before so in the middle of your sentence, try replacing so with “therefore” or “so that.” If your sentence seems to work with a replacement of “therefore” without changing the meaning of the sentence, then so is a coordinating conjunction and should have a comma before it. Let’s revisit one of our examples above.

Daniel had the highest score in math in the whole school, so he was made principal for the day.

Daniel had the highest score in math in the whole school, therefore he was made principal for the day.

The sentence still works, so we know that so is a coordinating conjunction here and is entitled to its comma. So that can be used in a similar way to confirm that so is being used as a subordinating conjunction.

I went to the store so I could buy tomatoes.

I went to the store so that I could buy tomatoes.

Because the substitution works, we know that there should be no comma in the sentence.

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