Grammar and Punctuation Rules

A comma (,) is a punctuation mark used to indicate a pause, separate parts of a sentence, or clarify meaning. It helps make sentences easier to read by separating ideas, items in a list, clauses, or even words.

As a general guide, commas are used when one would normally pause for a breath when speaking, and they are used to indicate general subdivisions within a sentence.

Separating items in a list: Commas are used to separate three or more items in a series.

• I bought apples, oranges, and grapes.
• She enjoys reading, cooking, painting, and hiking.

Before conjunctions or compound sentences: When two independent clauses are connected with a conjunction like for, and, nor, but, or, yet, or so (FANBOYS), a comma is needed before the conjunction.

• I wanted to go to the beach, but it started raining.
• She studied hard, so she passed the exam.

After Introductory Phrases or Clauses: Commas are often used after introductory words, phrases, or clauses that come before the main part of the sentence.

• After the meeting, we went for lunch.
• In the morning, I like to drink coffee.
• Actually, I think I'll take the purple one.

Setting Off Nonessential Information (Appositive/Parenthetical Phrases): Use commas to separate nonessential information that adds detail but isn't necessary for the sentence to make sense.

• My sister, who is a doctor, lives in New York.
• The movie, directed by Greta Gerwig, was a huge success.

With Coordinate Adjectives: If two adjectives equally describe a noun and can be swapped or connected with the word "and," they should be separated by a comma.

• It was a long, exhausting day.
• She wore a bright, colorful dress.

Separating Contrasting Elements: Commas are used when there is a contrast between two parts of a sentence, often with words like "but" or "yet."

• It was a beautiful day, yet there was no one at the park.
• I like tea, not coffee.

Dates, Addresses, and Numbers: Commas are used to separate parts of dates, addresses, and large numbers.

• He was born on July 11th, 2002.
• I live at 123 Main Street, Saint Paul, Minnesota.
• The car costs $50,000.

Comma Splice: A comma splice occurs when two independent clauses are joined with just a comma (without a coordinating conjunction or semicolon).

Incorrect:

• I wanted to go for a walk, it started raining.
• She loves to climb, high heights are her favorite.

Correct: 

• I wanted to go for a walk, but it started raining. (Using "but" as a conjunction)
• She loves to climb; high heights are her favorite. (Using a semicolon)

Omitting Commas in a List: Commas should be included between all items in a list of three or more. When the list has only two items, no comma is needed.

Incorrect:

• He bought apples bananas and grapes.
• She jumped over the hurdle, and the big rock.

 Correct: 

• He bought apples, bananas, and grapes.
• She jumped over the hurdle and the big rock.

Misplacing Commas with Nonessential Information/Parenthetical Phrases: Nonessential clauses or phrases should always be set off by commas, as leaving them out can make the sentence confusing or change its meaning. 

Incorrect: 

• My brother who lives in Chicago is coming to visit.
• That building which houses 247 cats needs to be repainted.

Correct: 

• My brother, who lives in Chicago, is coming to visit.
• That building, which houses 247 cats, needs to be repainted.

Commas are needed around the nonessential clauses/parenthetical phrases to set them apart from the main sentence, which indicates a slight pause or interruption in the flow of thought. Removing the parenthetical phrase should still leave the sentence grammatically correct and understandable:

• My brother is coming to visit.
• That building needs to be repainted.