The word so can have a few meanings and be used in several ways, and we will discuss all of them below. As with most rules on commas, some of them are grammatical, while some are stylistic, and thus more flexible. So, let’s start with the most clear-cut case of the use of so – as a conjunction.
So as a Conjunction
When so is used as a conjunction, it has the meaning of the reason or purpose for something, or the result of something. In this case, it follows the standard rule for commas and conjunctions:
- So is preceded by a comma if it connects two independent clauses.
- So is not preceded by a comma if it connects an independent and dependent clause.
Connecting an Independent and Dependent Clause
A dependent clause is a clause containing a subject and a verb, and provides additional information. It can modify the adjacent clause or serve as an additional component of an independent clause. Think of it like this – if it doesn’t make sense on its own, it’s a dependent clause.
When so is connecting an independent and dependent clause it does not need a comma before it.
Jacques went home so he wouldn’t listen to dungeon synth music. ✔
Jacques went home, so he wouldn’t listen to dungeon synth music. ✘
A good test is to switch so with so that. If the sentence still makes sense, it is connecting an independent and dependent clause, and you don’t need a comma before so.
I went to the pub so I could have a drink. ✔
I went to the pub so that I could have a drink. ✔
Connecting Independent Clauses
An independent clause is a clause that contains a subject and a predicate, and can form a complete sentence on its own. When so is connecting 2 of these clauses, you need to put a comma before so.
Jack and Jill went down the hill, so the witch put on Game of Thrones knowing she had some free time. ✔
Jack and Jill went down the hill so the witch put on Game of Thrones knowing she had some free time. ✘
Both the clauses Jack and Jill went down the hill and The witch put on Game of Thrones knowing she had some free time convey a complete thought and can stand alone. That’s what makes them independent clauses. You can use the same test from before. Replace so with so that and if the sentence doesn’t make sense, it is connecting two independent clauses.
I finished work early, so the boss threw a hissy-fit. ✔
I finished work early, so that the boss threw a hissy-fit. ✘
So as an Adverb
When so is used as an adverb it will usually be followed by an adjective or another adverb, and it can have a multitude of meanings. It can be used as an intensifier and for emphasis, for comparison, to refer back to something, etc. What is important here is that it does not connect two clauses.
In these cases, unless so is used to introduce a sentence (by itself or as part of an introductory phrase), you don’t need to put a comma before or after so.
They have so much rebuilding to do. ✔
They have so much, rebuilding to do. ✘
The Knicks lost? I thought so. ✔
The Knicks lost? I thought, so. ✘
So at the Beginning of a Sentence
This is where things get a bit complicated. What part of speech so is when used at the beginning of a sentence is not important (there are some complex discussions between linguists regarding this issue, but we won’t bother you with that); what we care about is how we use it. Obviously, you don’t need a comma before so in these cases, but a comma after so is another matter.
When you are using so to answer a question, you don’t need a comma after it.
Why did you cross the Rubicon? So I didn’t have to babysit Octavian. ✔
Why did you cross the Rubicon? So, I didn’t have to babysit Octavian. ✘
When so is used as an introductory unit it can have the meaning of well, or it can be an abbreviation of so tell me. In these cases, you need to put a comma after so.
So, you managed to get a new jersey, but forgot to buy milk again? ✔
So you managed to get a new jersey, but forgot to buy milk again? ✘
Take note that this is a common usage of so in informal speech. It is used to designate a pause that would normally exist if you were speaking and that makes the comma somewhat optional. Many speakers (this is not a strict rule, but even so) feel that beginning sentences with so in formal writing should always be off-limits. If you are writing a formal letter or scientific paper, avoid using it. If you do find it in formal writing, it is usually used as a conjunction or adverb and you don’t need a comma after it.
They asked for proof. So we explained heliocentricity. ✔
They asked for proof. So, we explained heliocentricity. ✘
So, to Sum Up
We have three distinct cases of the use of so:
- So as a conjunction
- So as an adverb
- So at the beginning of a sentence
In the first case, put a comma before so if it connects two independent clauses. If it connects an independent and dependent clause, no comma is needed. For the second case, when so is used as an adverb, you don’t need a comma before or after so.
The third and final case is when so begins a sentence. When you are answering a question or in formal writing, you don’t need a comma after so. In informal writing, where you would normally pause in speech, put a comma after so.