Grammar Girl, Mignon Fogarty, writes this:
Technically, those e-mail messages you write should begin Hi, John—with a comma after Hi.
You see, Hi, John is different from Dear John because hi and dear are not the same kind of word. Hi is an interjection just like wow and ugh, and dear is an adjective that modifies John.
In Hi, John you are directly addressing John, which means the punctuation rules of “direct address” apply. From a comma-rules standpoint, Hi, John is no different from Thanks for coming, John or Wow, John, what were you thinking?
You can end Hi, John with a period or, if you continue the sentence, a comma.
So now you know.
As you can see from the sentence in the headline, which appears in Hacker and Sommers’s Rules for Writers (7th edition), a missing comma can make for some pretty funny reading—even if we didn’t intend to be amusing.
For some people, though, commas are no laughing matter. Not long ago a friend of mine cried as she told me her tale of comma woe (which involved a mean-spirited professor, a red pen, and a classroom full of laughing students).
The rules regarding comma use are finite and are not too difficult to learn, though. Below are a few rules you might find especially helpful:
Use a comma between all items in a series (known as the Oxford, or serial, comma). Although some argue that a comma before the last item in a series is not necessary or, as might be the opinion of a newspaper editor, takes up too much space, I use it because it makes clear what otherwise might be confusing.
- CORRECT: I love my parents, Lady Gaga, and Humpty Dumpty (from Grammarly).
- UH-UH: I love my parents, Lady Gaga and Humpty Dumpty.
Use a comma before a coordinating conjunction (think FANBOYS—for, and, nor, but, or, yet, so) ONLY when what follows that conjunction is a complete sentence with a subject and a verb (an independent clause).
- CORRECT: I will cook Elmer, and I will also do the dishes.
- UH-UH: I will cook Elmer, and will also do the dishes.
Use a comma after an introductory clause or phrase.
- CORRECT: After taking a very long bus and train ride, my cousin said she was tired and wanted to go to bed.
- UH-UH: When Irwin was ready to iron his cat tripped on the cord (from Rules for Writers, 7th edition).
- EXCEPTION: You can omit the comma if the clause or phrase is short and the sentence will not be confusing without it. (Example—For once she was happy to see him.)
So now you know.