Being one of the most widely used punctuation marks, comma is probably one of the most misused ones as well. Writers frequently use commas whenever they would naturally make a pause, which is highly subjective. Below, you will find a summary of the most useful and simple rules on how to use commas correctly, as well some tips on how to avoid common mistakes associated with them.

SEPARATING ELEMENTS IN A SERIES

The simplest function of a comma is to separate individual elements (words, word groups) in a series of three or more.

David speaks English, French, and German

He will arrive today, tomorrow, or the day after that.

The last comma in the above series preceding and or or may seem peculiar. It is called the Oxford comma and is not obligatory. However, we certainly advise you to use it to avoid ambiguity:

I would like to thank my parents, William Shakespeare and Virginia Woolf.

Unless your parents are the two famous writers, which the construction above suggests, you need the Oxford comma before and Virginia Woolf to make it clear you are referring to three entities.

SEPARATING INDEPENDENT CLAUSES

Commas are also used to separate independent clauses (clauses that can stand on their own) connected by and, or, but, so, nor, etc. within a sentence:

She arrived home, but he was not there.

NOTE: The comma may be omitted if the subject does not stand in front of the second verb or in case of very short clauses:

He was aware of the problem and chose to ignore it.

She sings and he paints.

DEPENDENT CLAUSES

A comma is required if you start a sentence with a dependent clause (a clause that does not express a complete thought and thus cannot stand on its own).

If you have any questions, do not hesitate to ask.

NOTE: You do not have to use a comma if you start a sentence with an independent clause followed by a dependent one.

Do not hesitate to ask if you have any questions.

NONESSENTIAL ELEMENTS

Commas also separate or enclose what is considered nonessential information.

My co-worker, Mary, has just arrived to work.

John, believing his fiancée was unfaithful, broke off the engagement.

However, take a look at the following example:

My sister, Nancy, is a waitress.

The use of the commas implies that the author of the sentence has only one sister – she is sufficiently identified and her name is considered additional information.

My sister Nancy is a waitress. – Leaving out the commas, on the other hand, suggests that the author has more than one sister. Her name is essential information since it identifies which of the sisters is a waitress.

NOTE: Use commas only when an object or a person is adequately identified and you are providing additional information about it.

QUOTATIONS

Commas are used to mark the introduction as well as the interruption of a direct quote. They are used to separate the phrases such as she said or he asked, which identify the speaker.

Mary said, “I don’t believe you.”

“Why,” I asked, “don’t you believe me?”

MISCELLANEOUS

Use a comma:

  • after introductory elements, such as why, as well, hello, yes, etc. (Why, this is preposterous.),
  • before and after the following introductory terms if they precede a series of items – e., e.g., for instance, namely, that is, etc. (She bought a lot of things, namely, a dress, a skirt and five pairs of trousers.),
  • to enclose the elements interrupting the flow of the sentence, such as on the other hand, however, above all, nevertheless, etc. (She was, nevertheless, a good friend.),
  • in direct addresses to set off names, titles, nicknames, etc. (Mary, would you be so kind as to open a window? Farewell, Sir.),
  • before the expression ,
  • in dates of the format month-day-year before the year (February 10, 2020); in the format day-month-year a comma is not necessary (10 February 2020).

COMMON MISTAKES:

  • do not use a comma to separate a subject and its verb – you may be tempted to do that with long, complex subjects,
  • never use a comma before than when making a comparison (John speaks English better than Michael.),
  • when only the month and year are mentioned, there is no comma in between them (She was born in July 1990),
  • a comma is not sufficient to join two independent clauses – a conjunction (and, or, etc.) or a semicolon must be used:

Incorrect: The car was running low on gas, I drove to the gas station.

Correct: The car was running low on gas, so I drove to the gas station.