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