We all make mistakes. Here are some common grammar mistakes involving commas with explanations of the rules, so you can make fewer mistakes — at least in the grammar department.
Two complete sentences cannot be merged with a mere comma.
Incorrect: This song is awesome, it’s one of my favorites.
You can use a coordinating conjunction (and, but, for, nor, yet, or, so), a semi-colon (;), a dash (—), or a good old-fashioned period to split them up.
Correct: This song is awesome, and it’s one of my favorites.
Correct: This song is awesome; it’s one of my favorites.
Correct: This song is awesome—it’s one of my favorites.
Correct: This song is awesome. It’s one of my favorites.
However and therefore
The word “however” is one of the biggest accomplices in comma splices. It is not a coordinating conjunction like “and,” “but,” etc. “However” is a conjunctive adverb, on par with “therefore” or “furthermore.” As such, it cannot simply link two complete statements with a comma.
Incorrect: I wish it worked that way, however it doesn’t.
You can fix it the same way you would with any other comma splice.
Correct: I wish it worked that way; however, it doesn’t.
Correct: I wish it worked that way—however, it doesn’t.
Correct: I wish it worked that way. However, it doesn’t.
When “however” or “therefore” is used inside one complete statement, it should be set off with commas.
Correct: Therefore, I will write my sentences correctly.
Correct: I would be interested, however, to see how these grammar rules evolve.
Identifiers and [non]essential clauses
Commas are used to set off unnecessary details — these are called nonessential clauses. A nonessential clause gives information that doesn’t change the meaning of the sentence. Its nonessential, because you don’t need to know it.
These clauses need to be set off with two commas, or an opening comma and a period (if it’s the end of the sentence).
Correct: My husband, Hank, sells propane. (Correct because I only have one husband—I don’t have to include his name to identify which husband.
Correct: I went to visit my brother, John. (Correct if I only have one brother.)
Correct: Jenny Thompson, a senior, works with campus life.
No commas are needed when the information is necessary to clarify a part of the sentence.
Correct: My friend Angela is on the swim team. (I don’t need commas around Angela because I have more than one friend. I need her name to clarify which friend I’m talking about.)
Correct: I saw the movie Eternal Sunshine of the Spotless Mind with my friend Angela. (I didn’t include commas around the movie title because there are more movies than this one, and I have more than one friend — in both cases, you need to know the information.)
Correct: I saw the movie Eternal Sunshine of the Spotless Mind with my wife, Angela. (Like the above example, only now I used a comma before Angela because I can only have one wife, so her name isn’t necessary.)
Correct: Acclaimed author Henri Nouwen has dozens of books available.
In that last example, “acclaimed author” is the identifier. It needs no comma before the name. However, if the identifier follows the name, it does need commas.
Correct: Henri Nouwen, acclaimed author, has dozens of books available.
Here are some more examples of correct and incorrect identifier commas.
Incorrect: Henri Nouwen, author of The Inner Voice of Love and dozens of other books passed away in 1996. (Incorrect because the closing comma is missing.)
Correct: Henri Nouwen, author of The Inner Voice of Love and dozens of other books, passed away in 1996.
Incorrect: The film, The Artist, won the 2012 Oscar for best picture. (Incorrect because the film’s title is necessary to clarify which film.)
Correct: The Oscar-winner for best picture in 2012, The Artist, is a black-and-white silent film. (Correct because you don’t need “The Artist”—it is one and the same as the identifier that precedes it.)
Correct: An Omaha man, Paul Yoder, developed DonorElf. (Correct because you could omit “Paul Yoder” because of “an.”)
Correct: My dad, who lives for Thanksgiving, was devastated when the turkey burned. (Correct because the “who” clause is nonessential, so it’s set off with commas.)
For more information on commas, check out: