TL;DR:
Use commas in lists, after introductory clauses, before a conjunction joining two full sentences, and around nonessential information.
A comma alone cannot join two full sentences. That mistake is a comma splice.
When unsure, read aloud and listen for natural pauses.
Commas look small, but they change meaning. "Let's eat, grandma" and "Let's eat grandma" describe very different dinners. You do not need to memorise dozens of rules. Four cover most of what appears in school writing, and one common mistake accounts for many of the rest.
Rule 1: commas separate items in a list
When you list three or more things, put a comma between them. The comma before the final "and" is called the serial comma (or Oxford comma). Some styles require it and some leave it out, so follow what your teacher or style guide says. It prevents confusion in cases like "I thank my parents, Ana, and Max".
Rule 2: a comma after an introductory clause
When a sentence starts with a dependent clause or a phrase that sets the scene, a comma marks where the main sentence starts: "After the bell rang, the students left." If the main clause comes first, you usually do not need the comma: "The students left after the bell rang."
Rule 3: a comma plus a conjunction joins two full sentences
When two complete sentences are joined by for, and, nor, but, or, yet, so, put a comma before the conjunction: "I wanted to go to the park, but it was raining." The comma is there because both halves could stand alone. If the second half cannot stand alone, as in "I wanted to go to the park but could not", no comma is needed.
Rule 4: commas around nonessential information
Information that could be removed without changing the main meaning goes between commas: "My brother, who lives in Denver, is a nurse." This implies you have one brother. Without commas, "My brother who lives in Denver is a nurse" suggests you have more than one brother and are picking out the one in Denver. If the information is needed to identify the noun, no commas.
Worked example: fixing five comma errors
Here are five sentences, each with one error. Apply the rule, then check the result.
Original | Problem | Fixed |
|---|---|---|
We bought apples bananas and grapes. | Rule 1: list items need commas. | We bought apples, bananas, and grapes. |
After the bell rang the students left. | Rule 2: an introductory clause needs a comma. | After the bell rang, the students left. |
I wanted to go to the park but it was raining. | Rule 3: two full sentences joined by "but". | I wanted to go to the park, but it was raining. |
My brother who lives in Denver is a nurse. | Rule 4: nonessential information needs commas. | My brother, who lives in Denver, is a nurse. |
The test was hard, I studied for hours. | A comma splice: two full sentences with only a comma. | The test was hard, but I studied for hours. Or: The test was hard; I studied for hours. |
Test each fix. Sentence 1: three items, so two commas. Sentence 2: the clause "After the bell rang" cannot stand alone, so a comma follows it. Sentence 3: "I wanted to go to the park" and "it was raining" are both complete, so a comma comes before "but". Sentence 4: the Denver information is extra, so commas go on both sides. Sentence 5: a comma cannot join two full sentences. You have three options: a period, a semicolon, or a comma with a conjunction.
The comma splice
A comma splice happens when two independent clauses are joined by only a comma. Fix it with a period, a semicolon, or a comma plus a conjunction. Words like however and therefore do not join sentences by themselves, so "I was tired, however I kept going" is also a splice. Write "I was tired; however, I kept going."
Two more cases worth knowing
Direct address. When you speak to someone by name, set the name off: "Maria, please close the door." Without the comma, you would be talking about Maria rather than to her.
Dates and places. In a sentence, a comma separates parts of a date and an address: "She was born on June 3, 2010, in Chicago, Illinois." Notice the comma after the year and after the state when the sentence continues. Some style guides differ, so check yours.
These are not new ideas. Both are cases of setting extra information apart, like the fourth rule. If you keep that picture in mind, you will make the right call most of the time.
Where not to put a comma
Between a subject and its verb: "The student who studied hard, passed" is wrong.
After a conjunction in the middle of a sentence when it joins two verbs sharing a subject.
Between two adjectives that do not work independently.
Tip: Read the sentence aloud. A natural pause often marks a comma, but a pause alone is not enough. Always check against the four rules.
Your plan for this week
Take a page of your own writing and circle every comma. For each, name the rule: list, introduction, joining, or nonessential. Any comma you cannot name is a candidate for deleting. Then scan for places where two full sentences meet without any rule applying. If you want a second check, DuckyHelper can talk you through a sentence out loud, but the rule book for your class is the final word.
Frequently asked questions
Do I need the serial comma?
It depends on the style guide or teacher. Use one consistently, and add it whenever its absence would make a list ambiguous.
Is a comma always a breath pause?
No. That is a rough guide, but grammar, not breathing, decides where commas go.
Get started
