HighSchoolQ

✍️ English Grammar and Writing · Punctuation

Comma Rules: notes and common mistakes

Study notes

Key ideas

  • Most comma rules depend on clauses. An independent clause can stand alone as a sentence. A dependent clause cannot.
  • Seven standard jobs: a series (with the serial comma this site uses), after an introductory element, before a FANBOYS word that joins two independent clauses, around nonessential information, between coordinate adjectives, around direct address, and in dates and places.
  • Every rule has an exception: two items take no comma, a short opener may skip it, a FANBOYS word joining only two verbs takes none, essential information takes none, and cumulative adjectives take none.
  • A comma alone cannot join two independent clauses. Fix a comma splice with a period, a semicolon, or a comma plus a FANBOYS conjunction.
  • A single comma never separates a subject from its verb. Only a pair of commas around an interrupter can stand between them.
  • Commas follow grammar, not pauses. Decide by clauses, lists, and extra information.

Steps

  1. Find each subject and verb. Mark each clause as independent or dependent.
  2. If the sentence opens with a word, phrase, or dependent clause, put a comma after it. A very short phrase may skip it.
  3. If a FANBOYS word joins two independent clauses, put a comma before it. If it joins only two verbs or two phrases, leave the comma out.
  4. For any who, which, or that clause and any renaming phrase, cross it out. If the sentence still identifies the same person or thing, set it off with commas (a pair in the middle of a sentence, one comma at the end), and use which (or who for people), not that. If not, use no commas.
  5. In a list of three or more, put commas between the items, including before the last and.
  6. Test adjectives with and and by swapping them. Both pass: comma. Both fail: no comma. If they disagree, trust the and test.
  7. Scan for two independent clauses held together by only a comma, and fix each splice.
  8. Last, make sure that no single comma separates a subject from its verb.

Watch out

  • Do not put a comma before an essential that clause: The book that I borrowed was long.
  • Do not put a single comma between a subject and its verb, even a long subject.
  • Do not join two independent clauses with only a comma. Words like however, therefore, and then do not fix a splice when they start the second clause.
  • Do not put commas around essential information, and do not forget the second comma around extra information in the middle of a sentence.
  • Do not put a comma before and when it joins only two verbs, two phrases, or two items.
  • Do not put a comma between the last adjective and its noun.

Why it works

  • Commas tell the reader how the parts of a sentence fit together. A missing or extra comma can change the meaning: My brother, who lives in Ohio, plays the cello and My brother who lives in Ohio plays the cello say different things about how many brothers there are.
  • Careful punctuation helps readers trust your writing, in essays, applications, and emails.
  • Every comma rule rests on clauses, so learning clauses here also prepares you for semicolons, fragments and run-ons, and varied sentence structure.

Common mistakes

✗ MistakePut a comma wherever you naturally pause when you read aloud.
✓ InsteadPauses are an unreliable guide. You may pause after a long subject, where no comma belongs, and you hardly pause in Dallas, Texas, where one does. Decide by grammar: clauses, lists, and extra information.
✗ MistakeThe students in the back row, fell asleep. (A comma shows where the subject ends.)
✓ InsteadA single comma never separates a subject from its verb: The students in the back row fell asleep. Only a pair of commas around extra information may stand between them: The students, who were tired, fell asleep.
✗ MistakeThe book, that I borrowed, was long.
✓ InsteadA that clause is essential, so it takes no commas: The book that I borrowed was long. If the information is extra, use which with commas: The book, which I borrowed from the library, was long.
✗ MistakeThe bus was late, we missed the quiz. (The two sentences are closely related, so a comma is enough.)
✓ InsteadTwo independent clauses joined with only a comma make a comma splice, however closely related they are. Use a period, a semicolon, or a comma plus a FANBOYS word: The bus was late, so we missed the quiz.
✗ MistakeThe student, who finished first, won a medal. (Put commas around every clause that starts with who.)
✓ InsteadCommas go around a who clause only when it is extra information. Here it tells which student, so it is essential and takes no commas: The student who finished first won a medal.
✗ MistakeAlways put a comma before and.
✓ InsteadA comma goes before and when it joins two independent clauses or comes before the last item of three or more. It does not go before and when it joins only two verbs, two phrases, or two items.

Teach it back

Explain to a friend (or a pet!) how you can tell whether a comma goes before the word and in a sentence. Give one sentence of your own where it needs a comma and one where it does not.

Explaining it out loud shows what you really know. Did you…

  • I said that a comma goes before and when it joins two independent clauses, or comes before the last item in a series of three or more.
  • I said that no comma goes before and when it joins only two verbs, two phrases, or two items.
  • In my own pair of sentences, I pointed out the subject and the verb on each side of and to show why.

Your 3-week memory plan

  1. TodayRead the lesson, then try the flashcards.
  2. TomorrowRecall it from memory first, then check.
  3. In 3 daysTake the true/false and practice questions.
  4. In 1 weekMix it with older topics and teach it back.
  5. In 3 weeksA last quick review before it fades.

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