When to Use a Comma: 6 Comma Grammar Rules

A comma (,) is used in English to indicate a small pause. It’s a punctuation mark, used to separate independent clauses, items in a series, ideas within a sentence, and for certain number conventions. However, in writing you can’t use a comma to combine independent clauses or to end a sentence. 

When used correctly, commas help the reader to clearly understand the written message. However, errors can confuse meaning or convey a lack of professionalism, so it is important to understand how to use commas in your writing.

Keep reading to learn the common rules and mistakes writers encounter when using commas. Then, conveniently review your copy with the Originality.ai Grammar Checker.

When to Use Commas in English: 6 Basic Rules

1. Serialized lists

Commas are used to separate items within a list or series of three or more. Commas can be used to separate words, clauses, or phrases within a series. 

Examples:

• Series of words: I searched for the fastest way to walk, bike, or drive to school.
• Series of clauses: She studied consistently, she practiced every day, and she asked for feedback from her mentor.
• Series of phrases: The dog barked loudly, ran across the yard, and chased the squirrel.

In each of the above examples, a comma is used to separate the items within the series to show the clear distinction between each individual item.

2. Compound sentences

A compound sentence is made up of two independent clauses joined with a coordinating conjunction such as  ‘for,’ ‘and,’ ‘nor,’ ‘but,’ ‘or,’ ‘yet,’ and ‘so.’ Place a comma in front of the coordinating conjunction to separate the independent clauses. 

Examples:

• He finished his homework early, so he was able to watch his favorite show.
• We can go to the beach today, or we could visit a museum instead. 

3. Nonessential Clauses

Nonessential clauses are similar to what they sound like. They provide information but it’s not essential or necessary for understanding the main idea of the sentence or phrase. Nonessential clauses may also be called nonrestrictive clauses.

One way to test if a clause is nonessential is to read the sentence without it. If the sentence still makes sense and is understandable then that clause is nonessential.

You should separate nonessential clauses from essential clauses in a sentence using commas. 

• Middle of the sentence: Put a comma before and after the nonessential clause. 
• End of the sentence: Place one comma before the nonessential clause. 
• Tip: The relative pronoun “which” serves as a good indicator for nonessential clauses. 

Examples (nonessential clauses are italicized):

• My sister, who lives in New York, is coming to visit us next week.
• The film, released in 2023, was nominated for Best Picture.
• The restaurant is popular for its seafood, which is always fresh and expertly prepared.

4. Dependent introductory clauses

A comma is used after a dependent introductory clause. 

A comma signals the separation between the introduction of the sentence and the main idea; place a comma after the introductory clauses, phrases, or words. 

These introductions give additional details about the main idea of the sentence, but they are not independent clauses. 

Examples (introductions are italicized): 

• Introductory word: However, the rain grew heavier.
• Introductory phrase: Since the meeting was cancelled, we had time to finish our projects.
• Introductory clause: Because the sun was setting, the sky turned a beautiful orange.

5. Direct quotations

Direct quotations are used to represent spoken words or dialogue in a written text. If a speaker is identified, then a comma is used to separate the speaker and verb from the words that are spoken out loud and held in quotations. 

Examples (the speaker is italicized):

• “I love swimming in the ocean,” Andy said.
• “Summer is coming,” she stated, “The days are growing long.” 
• Daria yelled, “Time for dinner!”

6. Dates and numbers

Commas are used in writing conventions for dates and numbers. For dates, commas are used to separate the day from the year, both written in numerals and the day of the week from the month. 

Examples:

• Monday, August 24, 2020
• September 20, 1987
• Saturday, October 7

Then, another way that commas are used with numerals is for numbers that are longer than four digits. Adding a comma in this scenario is optional.

Examples: 

• 4,500 (four thousand five hundred)
• 10,000 (ten thousand)
• 3,000,000 (three million)

4 Comma Mistakes: When Not to Use Commas

1. Essential clauses

Essential clauses contain information that is necessary to understand the main idea of a sentence. An essential clause should never be separated using a comma (otherwise it gives the impression that the information isn’t necessary). 

One common source of confusion with commas in grammar is sentences that begin with ‘that.’ You don’t use a comma before the word ‘that’ in a sentence.

Examples:

• He was surprised that they remembered his birthday. 
• Jane couldn't believe that the concert was canceled at the last minute.

2. Separate a subject from its verb

A subject should not be separated from the main verb in the sentence. These errors occur more commonly in longer sentences. 

Examples:

• Incorrect: The happy dog, plays in the backyard all day.
  
  • Correct: The happy dog plays in the backyard all day.
• Incorrect: The cat, is sitting on the windowsill watching the birds.
  
  • Correct: The cat is sitting on the windowsill watching the birds.

3. Complex sentences

Another common comma mistake appears in complex sentences; don’t separate a dependent clause and an independent clause with a comma. The exception is if the dependent clause is an introductory clause or a nonessential clause. 

Examples (dependent clauses are italicized):

• If it rains tomorrow we will stay indoors.
• The teacher explained the assignment before the students began their work.

4. Comma splices

A comma splice is a term used for the error created when two independent clauses are joined with only a comma. 

A comma cannot join two independent clauses; a semicolon or a coordinating conjunction is needed to bind the two clauses together. 

Comma Splice Example: 

• Incorrect: Ali tracked his grades all semester, he was determined to improve his GPA.
• Correct: Ali tracked his grades all semester; he was determined to improve his GPA.

Final Thoughts

Commas are used in writing to bring clarity to complex and compound sentences by setting off nonessential information and separating distinct items in a list. 

Now that you have learned the basic rules for using commas, as well as some common errors to watch for, you are ready to employ commas confidently in your writing.

Streamline your editing process and catch common spelling, grammar, and punctuation errors with the Originality.ai Grammar Checker.

Read more grammar best practices in our top guides:

• What Is an Indirect Object in Grammar?
• What Are Action Verbs? How to use Dynamic Verbs in Writing
• What is The Direct Object in a Sentence?
• Stative Verbs: How to Use Them in Writing