A comma is a punctuation symbol used in multiple ways to indicate a pause or particular organization. Commas are one of the most useful tools.
Correctly used, they will make sentences clearer and improve your organization. Incorrect use, however, can cause confusing sentences or even create unintended meanings.
EXAMPLEI love eating crafts and pets.
Without commas to separate the items on this list, it seems that this speaker eats crafts and pets. Commas helps separate and organize the relationship between words and phrases in many kinds of sentences. Therefore, it’s not useful for a writer to just distribute them randomly, as some authors do.
Unless you know that a comma belongs in a sentence because there is a rule indicating so, it’s better to leave the comma out. In the above example, your reader will probably assume that you don’t eat pets and crafts, but that instead, you love eating, crafting, and pets.
A comma can play a number of different roles depending on where it is placed in a sentence. It's important to understand these placements so that you can ensure each of your commas is fulfilling the role you intend it to.
2a. In Lists
In a series or a list of items, commas are used to separate each item. When you have three or more entries in the list, then you have a series; if there’s just two, then it’s only a pair.
EXAMPLEI'm going to Nashville, Atlanta, and New Orleans.
Here you use a comma after the first item, Nashville, and then after each other item in the list, culminating in the final comma, which should go before the “and” or the “or” that will round out the list.
The same holds true when your series features items that are phrases, not singular words.
EXAMPLEMy kids need to brush their teeth, gather their backpacks, and eat breakfast before school.
In this sentence, each phrase serves as its own entry in the list.
2b. In Compound and Complex Sentences
Commas follow particular rules in particular sentence constructions, as well; they are very important in both compound sentences and complex sentences.
A compound sentence is a sentence that combines two or more independent clauses. A complex sentence is a sentence that combines at least one dependent clause with at least one independent clause. In both types of sentences, commas separate these clauses.
In compound sentences that are combined using coordinating conjunctions, the comma is placed before that coordinating conjunction.
EXAMPLEThe day is cold, but it's not snowing.
EXAMPLEI'm going to stay inside, so I won't need a heavy coat.
See how the comma in each of those sentences comes before the coordinating conjunction?
In complex sentences, the comma may separate the independent and dependent clause. However, this will only happen when the dependent clause comes first, not when the independent clause is first.
EXAMPLEA complex sentence where the independent clause comes first will look like this: We can't go skiing since it's not snowing.
But if you were to reverse the order and put the dependent clause first, then you would need a comma to separate the two.
EXAMPLESince it's not snowing, we can't go skiing.
Notice in that sentence how the comma separates the dependent clause and links it to the independent clause that comes second.
2c. In Introductory Phrases
Another place that you need commas is in introductory phrases. In these constructions, the comma sets off the introductory phrase, separating it from the sentence it’s going to introduce.
EXAMPLEOn snowy days, it's common for many people to work from home.
Notice here that the comma comes at the end of the introductory phrase, "on snowy days," and before the rest of the sentence.
When you read this out loud, think about where you would naturally pause. You wouldn’t say, "On snowy days it’s, common for many people to work from home," or "On, snowy days it’s common for many people to work from home." Those just don’t feel right.
2d. In Other Instances
There are some other places that you might find commas. For one, commas surround interruptions in sentences, which indicate that these interruptions are their own units of meaning.
EXAMPLEMy friends, who have never skied before, are planning a trip to Lake Tahoe.
EXAMPLEThe weather report, which is usually correct, indicates that we're going to see a lot of snow.
Similarly, commas set off dialogue or quotations, indicating that the words quoted or spoken are separate from the rest of the words.
EXAMPLE"I can't wait to build a snowman," said my sister.
EXAMPLEThe weather report says, "Snow will exceed four inches by morning."
You’ll also see commas in dates and geographic names. In the U.S., dates are indicated by using a comma to separate the month and day from the year.
EXAMPLEJanuary 1, 2016
In geography, you do the same to separate the city from the state it’s in.
At times, you might even find a comma in a place where you can’t think of any universal rule that makes it need to go there. And yet, that comma might be necessary for clarity. That’s okay; sometimes those commas are just necessary.
You’ll often find that you catch comma errors when you’re in the proofreading stage of the writing process, so be sure to read out loud.
Likewise, when you’re looking for places to add commas, you can read your sentences aloud to see where in natural conversational speech you might pause. That’s usually, though not always, a good clue that a comma belongs there.
Now consider this short paragraph.
Ski resorts, can be ideal locations for a family vacation. They are full of family-friendly activities. If skiing is too intimidating showshoeing can be fun. Everyone will enjoy the beautiful views and the warm fireplaces. We recommend Lake Tahoe's resorts for fun, family, and adventure this holiday season.
Is it missing any commas? Yes. First, it’s missing one here: If skiing is too intimidating, showshoeing can be fun.
You know this because when you read this part aloud, you find that you pause here. Additionally, if you look at the sentence structure, you can see that “if skiing is too intimidating” is a dependent clause subordinate to the independent clause, “snowshoeing can be fun.” Since that dependent clause comes first, there needs to be a comma separating the two.
Did you find any extra commas? Look at that first sentence, and read it out loud: Ski resorts, can be ideal locations for a family vacation.
You wouldn’t naturally pause there, so by reading aloud, you caught an extra comma that you can now remove.