1. Between a subject and its verb:
* Incorrect: The dog, barked at the mailman.
* Correct: The dog barked at the mailman.
2. Between a verb and its object:
* Incorrect: She ate, a delicious meal.
* Correct: She ate a delicious meal.
3. Between a preposition and its object:
* Incorrect: She walked, to the store.
* Correct: She walked to the store.
4. Between a coordinating conjunction (and, but, or, so, yet, for, nor) and the independent clause that follows:
* Incorrect: She went to the store, and bought some groceries.
* Correct: She went to the store and bought some groceries.
5. After an introductory phrase that is only a few words long:
* Incorrect: After the game, we went to dinner.
* Correct: After the game we went to dinner.
6. Before a dependent clause that begins with "that":
* Incorrect: She said, that she was happy.
* Correct: She said that she was happy.
7. Before a relative clause that is essential to the meaning of the sentence (restrictive clause):
* Incorrect: The woman, who lives next door, is a doctor.
* Correct: The woman who lives next door is a doctor.
8. In a series of three or more items when using conjunctions (and, but, or):
* Incorrect: She bought apples, oranges, and bananas.
* Correct: She bought apples, oranges, and bananas.
9. Between two adjectives that are not separated by a conjunction and do not modify the noun in different ways:
* Incorrect: She has a beautiful, red dress.
* Correct: She has a beautiful red dress.
10. After a salutation in a letter:
* Incorrect: Dear John,
* Correct: Dear John,
11. After the closing of a letter:
* Incorrect: Sincerely,
* Correct: Sincerely,
Important Note: These are just some of the common rules. There are exceptions, and the best way to learn proper comma usage is to practice and consult a grammar guide.