Use commas when main clauses are connected by any of the seven coordinate junctions (and, but, for, or nor, so, yet).
Examples:
I would have arrived on time, but I got stuck in traffic.
Sam feels terrible because she was not promoted, yet she continues to slack off.
The doctor was not available yesterday, so I asked for another appointment.
2. Use commas after introductory elements such as clauses, phrases, or words. These are words that appear before the main clause to prepare the readers for the meat of the sentence.
a. Typical words for introductory elements that must be followed by a comma are when, while, because, if, as, since, after, although.
When all attendees arrive, we’ll kick off the meeting.
While I was away on a trip, the burglars broke into my house.
Because she overslept, she missed the train.
Exception:
When a dependent clause (A clause with a subject and a verb but does not deliver a complete thought.) follows a main clause, there is no need to use a comma except for cases of extreme contrast.
Incorrect: I laughed, when I heard the news.
Correct: I laughed when I heard the news.
Incorrect: I had to rush back home, after the concert.
Correct: I had to rush back home after the concert.
Exception:
When the dependent clause starts with an adverb of concession, a comma is needed. The adverb of concession expresses an idea that is contrary to the main idea.
Example:
He spoke with conviction, although most people in the audience were not persuaded.
b. Participial and infinitive phrases, absolute phrases, nonessential appositive phrases, and long prepositional phrases (more than four words) are some typical introductory phrases that must be followed by a comma.
Having completed the project, he took on a new one.
To secure a parking space, you’d better arrive early.
After the match but before dinner, we went for a walk.
c. Yes, however, and well are common introductory words that must be followed by a comma.
Examples:
Yes, the apartment should be ready by tomorrow.
However, they may not enter the building premises.
Well, I think it is necessary.
3. To separate clauses, phrases, and words that are not vital to the main idea of the sentence, we use a pair of commas in the middle of a sentence.
Clause: That man, who lives next door to me, is the team football coach.
Phrase: The teacher sets high standards for his students. The school, on the other hand, is rather indifferent.
Word: We understand your complaint. In this situation, however, it lacks merit.
Notice that when you omit or change the position of the clauses inside the comma, the sentence still makes sense. These clauses, phrases, and words are nonessential and therefore requires the use of a pair of commas.
4. In contrast to rule no. 3, we should not use commas to separate essential elements of the sentence. These clauses usually begin with the word that. When used after nouns and following a verb that expresses mental action, that clauses are always essential.
That clauses after nouns:
The table that you bought isn’t sturdy.
The bananas that we harvested are overripe.
That clauses after a verb expressing mental action:
She is hoping that she can travel again soon.
Marta is dreaming that she can marry the man of her dreams.
I wish that this pandemic would soon be over.
5. To separate three or more words, phrases, or clauses in a series requires the use of commas.
The five dominant political parties in Spain at present are Podemos, PSOE, Ciudadanos, Partido Popular, and Vox.
This pandemic brought about high unemployment rate, medical crisis, economic collapse, and heightened xenophobia.
The manager, who is responsible for his department, who has all the resources needed for the project, and who is aware of the project deadline, has failed to achieve the objective.
6. Coordinate adjectives are two or more adjectives of equal value that modify the same noun. Use a comma to separate these coordinate adjectives. Do not use a comma between the last adjective and the noun. Similarly, do not use a comma with non-coordinate adjectives.
There are two ways to spot coordinate adjectives. These are:
1. When the sentence still makes sense if the adjectives were placed in different orders.
2. When the sentence still makes sense if ‘and’ is used between the adjectives.
Examples of coordinate and non-coordinate adjectives:
We were entertained by the melodious, distinct music. (coordinate)
She has two tiny Terrier dogs at home. (non-coordinate)
Non-coordinate adjectives are not separated by a comma and are not of equal value. As a rule of thumb, adjectives of size are stated first then adjectives of age, color and material should follow.
Examples:
Jose bought three blue pairs of shoes.
You have to pass by a dusty dirt road.
7. To separate contrasted coordinate elements or to imply a pause or shift.
She was only suggesting, not imposing.
We have gone this way before, haven’t we?
The man seemed oblivious, even perplexed.
8. To separate phrases at the end of the sentence that refer to any other parts of the sentence. These phrases are free modifiers that can be placed anywhere in the sentence without confusing the main thought of the sentence.
Example:
Marta smiled indulgently at the dinner table, waiting calmly for the food. (correct)
Incorrect: Ana smiled at Marta, waiting calmly for the food. (Who is waiting, Ana or Marta?)
Waiting calmly for the food, Ana smiled at Marta. (correct)
Ana smiled at Marta, who was waiting calmly for the food. (correct)
9. To separate all geographical names, items in dates (excluding the month and day), addresses (excluding the street number and name), and titles in names.
The corporate headquarters is in Alcala de Henares, Madrid, and is in the northeastern part of the community.
On July 14, 1776, the US Declaration of Independence was adopted.
Alejandro Gonzales, Ph.D., is handling this class.
If only the month and the year is stated, a comma is not necessary after the month or year.
Example:
The global unemployment rate rose in March 2022 due to the COVID-19 pandemic.
10. To switch between the main discourse and a quotation.
Luisa replied bluntly, “You have no business in this matter.”
“They have finally, “he said proudly, “completed the task long before the deadline.”
11. Wherever necessary to avoid potential confusion or misreading.
To Javier, Manuel is a true leader.
Common mistakes in using commas.
1. Separating the subject from the verb.
Incorrect: A fourteen-year-old in Estonia, is allowed to get married.
Incorrect: One of the key skills of a business analyst, is understanding the business objectives.
2. Separating the two verbs or verb phrases in a compound predicate.
Incorrect: We prepared the food and drinks, and began to watch the movie.
Incorrect: I walked past the alley, and bumped into an old friend.
3. Separating two nouns, noun phrases, or noun clauses in a compound subject or compound object.
Incorrect (compound subject): The business developers from your department, and the account managers from mine are teaming up for this event.
Incorrect (compound object): She assured me that the house is still available, and that the property owner wants to speak with me.
4. When a dependent (subordinate) clause comes after the main clause except for cases of extreme contrast.
Incorrect: The cab driver arrived, while I was running down the hall.
Correct: He was not a very accommodating host, although he invited almost everyone to the party. (This is an example of extreme contrast.)