The wide range of comma rules makes it difficult to know all the rules without consulting a grammar handbook. When handbooks are not available, it is useful to be able to categorize these rules in order to provide a general idea of the four most common situations that require commas: extra material, lists, introductory elements, and compound sentences.
This comma use refers to three different situations:
Parenthetical Expressions: words or phrases that often interrupt the sentence and are not part of the independent main clause.
Example: My brother, believe it or not, loaned me $2,000.
The phrase “believe it or not” is extra material; if you eliminate it, what remains is still a complete sentence. Therefore, put commas on both sides of this extra material.
Appositive Phrases: phrases that define or give more information about a noun.
Example: My dog, the muddy one on the carpet, loves to dig holes.
The appositive phrase “the muddy one on the carpet” is also extra material. Put commas on both sides of it.
Conjunctive Adverbs: adverbs that occur in the middle and the end of sentences as extra material.
Example: My teacher, however, did not believe my excuse.
My teacher did not believe my excuse, however.
This comma rule applies to sentences where a series or list of items is given:
Example: I went to the store to buy milk, eggs, and bread.
I went to the store to buy milk, eggs and bread.
Either way is acceptable if meaning is clear, but be consistent with the pattern you choose.
To prevent misreading, use commas to set off common introductory elements from the main part of the sentence:
Dependent Clauses, which begin with subordinating conjunctions (e.g., when, since, after, although);
Example: When I was a child, I liked to ride my bike.
Example: Although Susan called, John did not get the message.
Infinitive Phrases, Long Prepositional Phrases, and Participial Phrases;
Example: To tell the truth, we made the wrong decision.
Example: Before the important business meeting, she had to make her own
Example: Arriving very early for work, she found the parking lot empty.
NOTE: No comma is needed if the introductory phrase is short and closely related to the main clause, and the meaning is clear.
Example: After dinner we left.
and Conjunctive Adverbs when placed at the beginning of a sentence (e.g., however, indeed, furthermore, therefore).
Example: However, I also like to watch television.
Example: Furthermore, he wanted action to take place by next week.
Use a comma to separate the independent clauses in a compound sentence:
Example: The snow started to fall heavily, so all the schools and
universities closed early.
The comma is optional if both independent clauses are short and the meaning is clear.
Example: John is tall so he ducks when entering rooms.
NOTE: When the same subject is doing both actions (a compound verb but not a compound sentence), do not use a comma.
Example: She skips and runs down the sidewalk.