Grammar Guide: When to Use a Comma

Nothing gets us in trouble more as a writer than misusing punctuation. A misplaced comma can

change the meaning of what we are saying. To help you understand the basic rules, I am going

to keep the sample sentences very basic.

First rule: Any part of a sentence that can be deleted without changing the basic meaning of

the sentence should have commas before and after that word, clause, or phrase.

Example: 

Oslo, who is my smallest dog, is learning how to sit. Identify the core sentence: Oslo is

learning how to sit. The fact that he is my smallest dog is a clause that can be deleted without

changing that core sentence. This additional information is just a bonus for the reader. The

clause needs to be separated by commas.

If the wording being removed changes the meaning of the sentence, no commas are needed.

Example: 

Dogs who are afraid of water wouldn’t like to swim in my lake. No commas should be

put around who are afraid of water because that is key to the sentence. Without that

information, we would only have this: Dogs wouldn’t like to swim in my lake. That isn’t true,

however. Lots of dogs do swim in lakes—just not dogs who are afraid of water.

We could change the sentence, though, to give a specific example (and a clause which needs

commas: My brother’s dog, who is afraid of water, doesn’t like to visit my lake. the core

sentence is true without the clause about water: My brother’s dog doesn’t like to visit my lake.

Note for ACT test takers: clauses that start with “that” don’t take a comma; clauses that start

with “which” take commas.

Example: Maps, which can be very difficult to read without glasses, were handed out to

travelers. Maps that fit easily into coat pockets are too small for me to read.

Note: There is an exception to the rule, of course. Pronouns that are used in a sentence for

emphasis are not separated by commas even though the sentence would stand alone without

them.

Example: The governor himself came to our graduation. I myself was surprised.

Second rule: Interruptions/interjections in a sentence need to be set apart by a comma.

Anything that breaks the flow of the core sentence could be deleted without changing the

meaning.

Example: 

However, I still went to the library.

I still went to the library, however.

I, however, still went to the library.

Third rule: Introductory words, phrases, or clauses should be set apart with a comma. This

introduction to a sentence could be the name of a person being addressed or could be a

dependent clause giving the reader additional information. Any opening clause or phrase needs

a comma after it.

Examples: 

Even though I was late, I still arrived in time to see the fireworks.

John, are you coming to class Tuesday?

In the winter, I like to drink hot cocoa.

As crazy as it sounds, we like to camp in really cold weather.

Fourth rule: Commas are used to connect independent clauses (that could stand alone) if they

are combined into one sentence using FANBOYS (for, and, nor, but, or, yet, so). These

coordinating conjunctions require a comma to be placed before them.

Example: 

I wanted to drink tea for lunch, but my sister drank it all before I got home.

I am saving up for a new bike, so I applied for the job at the library.

My family bought a cabin, and we are going to stay there this weekend.

Notice that these are all independent clauses. They could each stand alone without the

conjunctions:

I wanted to drink tea for lunch. My sister drank it all before I got home.

I am saving up for a new bike. I applied for the job at the library.

My family bought a cabin. We are going to stay there this weekend.

Note: “So” is a tricky conjunction. If the clause with it is necessary for the meaning of the

sentence, no comma is used.

Example: 

I woke up early so I could finish my homework. (If you could read this as “so that,” no

comma is needed. You are telling someone why you woke up early. If you could use the word

“therefore” instead of “so,” you use as comma. See the example above.)

Fifth rule: Use commas to separate items in a list.

Example: 

I bought sugar, eggs, and flour to bake my desserts.

Jenny, Mark, Paul, and Carol are coming to dinner.

I am going to do my homework, eat dinner, and go to bed early.

The ACT follows the Oxford comma rule. A comma is used before the “and” preceding the last

item in the list.

If multiple adjectives come before a noun and the order they are listed in doesn’t matter, you

need to separate them with a comma.

Example: 

I want a green, leafy bush for the front flower bed.

I adopted a young, energetic puppy.

If the order matters for descriptive words, no comma is needed.

Example: 

My first math teacher has retired. (First and math are both descriptive words.

However, it would be incorrect to say: My math first teacher has retired.)