Besides the period, the comma is probably the most widely used punctuation mark in English writing. In fact, the comma might actually be used more frequently than the period, as some sentences have multiple commas but only one period (like this one).
In this post, I want to go over five common comma errors found in student writing and how you can fix them.

1. No Comma After Introductory Clause

Of all the uses for a comma, this is probably the biggest. It’s also one of the most forgotten by students.

A comma is necessary after an introductory clause to notify the reader that the introductory element has come to an end. Without a comma, sentences can be confusing and difficult to follow.

• After eating your brother will do the dishes.
• After eating, your brother will do the dishes.

As you can see, the second sentence is much more clear and logical—and less frightening.

2. Commas Setting Off Restrictive Clauses

Another common mistake is to use commas to set off restrictive clauses.

A restrictive clause is a clause that tightens or restricts the meaning of a sentence, making it more specific. These clauses are sometimes called essential clauses.
Setting a restrictive clause off by commas, however, can drastically change the meaning of your sentence.

• Basketball players who win championships should be paid millions of dollars.
• Basketball players, who win championships, should be paid millions of dollars.

We can clearly see how these sentences differ in meaning. The first sentence says only the basketball players who win championships should be paid millions of dollars. The second sentence suggests that all basketball players should be paid millions of dollars, regardless of the amount of championships they win.

3. Comma Splice

The comma splice affects students and non-students alike, but the solution is simple. A comma alone cannot join two independent clauses.

• The beach was nice today, the water was warm.

This sentence needs to be split up or combined with a coordinating conjunction.

• The beach was nice today, and the water was warm.

4. No Comma Before Coordinating Conjunctions

When joining two independent clauses with a coordinating conjunction, you must use a comma before the conjunction. The comma tells the reader that one independent clause is about to stop and that one is about to begin.

• The sun was bright but I brought my sunglasses. Incorrect
• The sun was bright, but I brought my sunglasses. Correct

5. Commas Setting Off Restrictive Appositives

An appositive is a noun or noun phrase that renames a nearby noun.

• The Lord of the Rings, a fantasy novel, is a worldwide favorite.

In this sentence, the phrase a fantasy novel is acting as an appositive. It renames the book title. Commas set it off because it is a nonrestrictive appositive.

As is the case with nonrestrictive and restrictive clauses, so it is with appositives: nonrestrictive appositives are set off commas; restrictive appositives are not.
If you set off a restrictive appositive with commas, the meaning of your sentence will change.

• We listened to a symphony by the world famous Russian composer, Rachmaninoff.

As this sentence currently reads, it makes it sound as if there is only one world famous Russian composer, which clearly isn’t the case. The comma should be removed.

• We listened to a symphony by the world famous Russian composer Rachmaninoff.

For a free writing cheat-sheet on common writing mistakes, please check out my e-book 35 Mistakes to Avoid in Your Writing. For full explanations on many of English’s most confusing words, visit the Easily Confused Words section at Writing Explained.