Commas can be beastly. In this post, I'll cover some of the most common scenarios where people run into trouble with commas.
Don't use a comma to separate the subject and predicate
The subject of the sentence describes who or what the sentence is about. The predicate describes the action of the subject. For example, in the sentence "Sandra went to the park," "Sandra" is the subject, and "went to the park" is the predicate.
Sometimes, I see writers put a comma between the subject and predicate, which is incorrect. For example:
Sandra went to the park.
The writers and editors are learning to use commas.
Sandra, went to the park.
The writers and editors, are learning to use commas.
Addressing someone specific
You do put a comma after the subject if you are addressing the person directly, such as in an email:
Sandra, have fun at the park today!
As another example, both of these sentences are correct and mean two different things because of the comma:
Writers, take every opportunity to learn about commas. (I'm addressing the writers directly and am telling them to go learn about commas.)
Writers take every opportunity to learn about commas. (I'm making an observation that writers in general are driven to learn about commas.)
Separating a dependent clause from an independent clause
Let's get into something a bit more complex. Clauses have a subject and a verb. An independent clause forms a complete thought and can stand on its own as a sentence:
Sandra went to the park.
A dependent clause (also called a subordinate clause) provides more information about the independent clause and cannot stand on its own as a sentence. Consider this dependent clause:
When Sandra went to the park
When she went to the park...what? What happened? Because it starts with the dependent word "When", it's not a complete thought on its own and needs an independent clause to finish the thought:
When Sandra went to the park, a pack of wild dogs appeared.
Now we have the dependent clause (When Sandra went to the park) followed by the independent clause (she saw a pack of wild dogs), and the thought is complete.
Notice that there is a comma between the two clauses. Here comes the tricky part: when the dependent clause comes first, use a comma, but when the independent clause comes first, don't use a comma. (English is ridiculous sometimes.)
Using our previous example, the dependent clause is first, so there's a comma. If we reverse the clauses, though, you do not use a comma:
A pack of wild dogs appeared when Sandra went to the park.
Why do we need the comma when the dependent clause is first? Well, consider this sentence:
If you didn't have the comma, a reader would see this at the beginning of the sentence:
Before configuring the server users
Can you see how readers might think this sentence is talking about server users? If so, readers will become confused when they get to "must download" and will pause and reread the sentence. And that breaks the hiccup rule, which is a big no-no in technical writing.
Using that vs. which
This is an important topic that happens to fall within the discussion of commas, so let's tackle it!
Figuring out when to use "that" or "which" before a clause can cause writers to tear their hair out. Here's how it works: if the information is essential to understanding which thing you're talking about, use "that". For example, if you have two cars, and you want to tell me about the car in the garage, not the car in the driveway, you would write:
The car that is in the garage is broken down.
This is an example of a restrictive clause, also called an essential clause, because it's required in order for the reader to know which car you're talking about. Use "that" to introduce restrictive clauses.
However, if you only have one car, and you are simply mentioning its location as an aside, you would write:
The car, which is in the garage, is broken down.
This is an example of a nonrestrictive or non-essential clause. The information is not required for the reader's understanding and is just additional information.
Notice that when you use "that", you do not use any commas, whereas when you use "which", you surround the phrase with commas. Look at what happens if I omit the commas in the second example:
The car which is in the garage is broken down.
Now, the reader has no way of knowing whether "which is in the garage" is essential (there are multiple cars, and I'm talking about the one in the garage) or non-essential (there's only one car, and it happens to be in the garage). So whenever you use "which" to introduce a non-essential clause, be sure to surround the clause with commas.
If you're talking about a person instead of a thing, use "who" instead of "that" or "which", and use commas to indicate whether it's essential or non-essential information. For example:
The agent who read my book called me yesterday. (I sent my book to multiple agents, but only one read it, and she's the one who called me).
The agent, who read my book, called me yesterday. (I sent my book to only one agent, and she called me yesterday. As an aside, she actually read my book!)
You can see a nice blog post entirely devoted to restrictive and non-restrictive clauses over at the Grammerly Blog.
Using serial commas
People are damn passionate about this topic, so I'm about to alienate half my readers. Into the fray!
The serial comma, also called the Oxford comma, appears after the second-to-last (penultimate) item in a series. For example:
The server receives the request, sends the response, and logs the activity.
That comma before "and" is the serial comma. Because it can make a difference in meaning, we always use it in technical writing, and I use it in my creative writing as well. However, AP Style dictates that you do not use a comma after the penultimate item, so you'll notice its absence in news articles, marketing content, etc. So if a marketing piece were to write the same sentence, they'd usually write:
The server receives the request, sends the response and logs the activity.
This might not seem like a big deal until you consider more complex sentences or a series where the word can mean two different things. For example:
Every day after work, he drinks a beer, eats a burger and fries an egg.
Did you pause for just a moment after "fries", because you were grouping "burger and fries" in your mind and thought "fries" was referring to french fries? There goes the hiccup rule! With a serial comma, there's zero confusion:
Every day after work, he drinks a beer, eats a burger, and fries an egg.
Let's look at another example:
In his speech, he thanked his parents, Elvis and Madonna.
This makes it sound like Elvis and Madonna are his parents. With a serial comma, though, it all becomes clear:
In his speech, he thanked his parents, Elvis, and Madonna.
To see a really hilarious (and not quite safe for work) illustration that drives this point home, check out this Verbicide post. Read the comments if you want to see how worked up people get over this topic.
Commas are very important for communicating clearly. When used correctly, they break up complex sentences, set off non-essential information, and help avoid the dreaded hiccup rule.
In the next post, Using Commas - Part Two, I'll talk about more pitfalls with commas. But for now, let's take a pause.