Comma Rule #2: Between Independent Clauses
This rule does require some grammar speak because we have to understand basic sentence structure. Don't be overwhelmed though! I assure you, these are all things you intrinsically understand as an English language writer. Now, it's just about learning the terminology.
What is an Independent Clause?
An independent clause is a chunk of writing that can stand on it's own--it's independent!
Without any help from anyone else, it is a complete thought, sometimes even a complete sentence.
How do we know something is an independent clause? It needs to have a Subject and a Verb. Subjects and verbs go together like peanut butter and jelly.
Subject: something (usually a noun or pronoun) that performs an action.
Verb: the action the subject performs.
Let's look at an example: Shakespeare wrote plays.
Is it an independent clause? Subject: Shakespeare. Verb: wrote. It is! Sometimes it is easiest to find the subject first, and sometimes it is easiest to find the verb first. Either way, figuring out who is doing what is how you identify the parts of a clause.
What Happens When Independent Clauses Join Up?
Some sentences contain more than one independent clause. Have you ever had a teacher or editor tell you that you have run-on sentences or comma-splices? That just means you were trying to fit two independent clauses in your writing without connecting them in some way.
One of the easiest ways to connect independent clauses is a comma and a conjunction.
Let's define another grammar term: Conjunctions.
As a part of speech, conjunctions are just connectors. They show relationship between ideas.
You may have learned about conjunctions using the mnemonic FANBOYS.
F - For
A - And
N - Nor
B - But
O - Or
Y - Yet
S - So
If you have two independent clauses in the same sentence, figure out which conjunction shows their relationship. The next step is just adding a comma before the conjunction and you have created a compound sentence.
Let's look at our example again and add another clause: Shakespeare wrote plays [ ] he founded a theatre company.
From the list above, let's use "and" as our connector, so if we add that in with the proper punctuation we get: Shakespeare wrote plays, and he founded a theatre company.
A Word of Caution
Conjunctions don't always get a comma before them. Connecting independent clauses is just one of the many jobs they can perform in a sentence.
As you are editing and checking to you have commas and conjunctions between your independent clauses, be careful that you don't accidentally break up a subject and verb couple. Remember, if both halves of the sentence don't have both a subject and a verb, then it isn't an independent clause and this comma rule doesn't apply.
Shakespeare wrote plays, and he founded a theatre company. Both halves of our sentence have a subject and verb (Shakespeare / Wrote and He / Founded), so both are independent which means they get a comma!
Shakespeare wrote plays and founded a theatre company. In this rewrite, there is no subject for the verb "founded," which means the second clause is not independent, so the sentence does not need a comma before "and." The subject verb couple is Shakespeare / founded, and a comma in between them would break them up.*
*This doesn't mean you can never have a comma between a subject and verb couple; it just means you shouldn't have a comma-conjunction combination between a subject and verb couple.