Listing

We use the comma to separate words, phrases or clauses in a series.
  • She went to Safeway to pick up bread, milk, and cookies.
  • I looked for my keys on the counter, under the bed, behind the sofa and in my pockets
  • I brought the movies, Sarah made popcorn, Tony made some iced tea, and Lisa did very little.
For items to be separated only by commas, there should be at least three items in the series. An exception to this is a series of adjectives:
  • He was a humble, gracious champion.

Linking

We use the comma to link two independent clauses with a coordinating conjunction (and, but, so, or, for, nor, yet)
  • I went to Paris for my holiday, but I didn’t go to the top of the Eiffel Tower.
  • He has studied English for years, yet he cannot speak a word of it.

Introducing

We generally put a comma after an introductory word, phrase, or clause.
  • However, I did not enjoy the movie as much as everyone else did.
  • Because of the storm, the flight was delayed.
  • Because he was late for the meeting, we had to tell him what he had missed.

Adding and Interrupting

We use commas to add non-essential words, phrases or clauses into the middle, or at the end, of a sentence.
  • I, however, do not agree with the author's views of contemporary culture.
  • The presentation was, in my opinion, too long overall and unclear at points.
  • A screwdriver, a mixture of vodka and orange juice, was his father’s drink of choice.
  • Toronto, which is Canada’s largest city, is often mistakenly thought to be the nation’s capital.

Writing Dates

We use commas to separate information in dates.
  • The Beatles first appeared on American television on February 9, 1964.
When the date appears in the middle of the sentence, there should be a comma before and after the year:
  • The Beatles' performance on the Ed Sullivan Show on February 9, 1964, was their first appearance on American TV.
If the date contains only the month and year, but not the specific day, no commas are necessary:
  • He was actively involved in these humanitarian efforts from August 1971 to June 1975.

Writing Addresses

We use commas to separate information in an address.
  • The college campus is located at 1050 Alberni Street, Vancouver, BC.

Quoting 

We use commas to set off a quotation within a sentence.
  • “Your final presentations are due at the end of the week,” he said to the class.
  • “If you are hoping to get a good score on your presentation,” the teacher said, “then I suggest you practice your pronunciation beforehand.”
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