A clause, at the very minimum, has a subject and a verb. If the clause can stand by itself as a sentence, it is an independent clause. To join two independent clauses into one sentence, place a comma instead of a period at the end of the first sentence, add a coordinating conjunction and then write the second clause. Example: “John went to town, but Joe stayed home.”
When you introduce your main sentence with a phrase or with a dependent clause, which contains a subject and verb but is not complete by itself, place a comma after the phrase or dependent clause. Phrase example: “Due to illness, Joe did not go with John.” Clause example: “When John went to town, he drove the car.”
Do not insert a comma if the main clause comes before the dependent clause.
Use a comma after the main sentence just before a participle phrase that modifies something in the main clause. Example: “He turned away, having embarrassed himself thoroughly.”
Except when AP Style is required, place a comma after each word or phrase if there are three or more in a series. For AP Style, leave out the final comma that normally would come before the last word or phrase.
Clauses that give additional information but are not essential in narrowing or identifying the subject you are talking about are nonrestrictive clauses. They often begin with “who” or “which.” Place a comma before and after such clauses. Example: “My car, which is blue, gets good gas mileage.”
Do not insert commas with a restrictive clause. Example: “Children who eat a nutritious breakfast are more alert in school.”
Words that call by a different name a noun or pronoun that was just mentioned are called “appositives.” Appositives are set off by commas from the rest of the sentence. However, if the words serve to clarify the previous noun or pronoun and are essential to a correct understanding of the sentence, do not use commas. Examples: “My dog, Rex, loves to play.” (I only have one dog.) “My dog Rex loves to play.” (I have more than one dog. Rex is the one that loves to play.)
When you include phrases in the middle of your sentence that break the flow of the sentence, set them off with commas to alert the reader. Example: “I thought last night, full of wind and moonlight, would be good for flying my kite.”
Test two adjectives by saying “and” between them. If they still make sense, place a comma after the first adjective. For example: “The sad, tired boy fell asleep.”
Read your work once you have inserted commas to see if it flows well. Check for understanding.
If a sentence seems too long, break it into two sentences.
Make sure you have not inserted a comma that, instead of separating interrupters or appositives, separates the subject from its verb.