Comma rules can be confusing, not necessarily because they're difficult, but because people often disagree on what they are. One teacher may tell you one rule for using commas while another may tell you the opposite, and the frustrating truth is that they're probably both right.
The rules for comma use are, for the most part, firmly set, but there are a few gray areas. It's helpful to know what they are so that you understand where you absolutely need a comma, where you absolutely shouldn't put one, and also where you can fiddle around a bit.
1. Use a comma to separate items in a list of nouns. If you have more than two nouns, you need to separate them with commas. In a list of nouns, you will also separate the final two with the word "and" or the word "or" like this:
Most people consider it correct to include a comma after "lemons" (called the Oxford or serial comma), but it's not necessarily incorrect to leave it out. It's up to you. If you feel like a comma is needed there to avoid confusion, use one. If you prefer minimal punctuation and feel the sentence is readable without it, don't use one.
2. In a string of two or more adjectives use a comma. If the adjectives come before the noun, don't use "and." You only need to use "and" in a list of adjectives if the list comes after the verb "to be." Look at the following sentences:
3. Don't separate a subject from its verb with a comma. Even if the subject is very long and you feel a comma is needed to allow the reader to pause for breath, don't do it.
4. Use a comma to separate clauses, both dependent and independent.
As in the first rule, the final comma is not necessary in a string of dependent clauses. However, as the clauses get longer, leaving out that last comma can get confusing, so it is often better to put it in just to clear things up:
When your clauses are independent (they could stand alone as complete sentences), it is absolutely necessary to use both the comma and the "and" before the final one:
5. Use a comma to set off a non-defining subordinate clause or an appositive. A non-defining subordinate clause gives some information about a noun, but the information is not necessary for identifying that particular noun. These clauses usually begin with "which" or "who."
The non-defining subordinate clause, "which was built in 1930" gives some information about the Empire State Building, but we don't need that information to identify the building or distinguish it from any other Empire State Buildings. It's just extra information.
An appositive is similar, but it doesn't include "which" or "who." It's a word or phrase that can be substituted for a name.
Here, you could identify Phyllis's husband as either "Bob Vance" or "the president of Vance Refrigeration." They are the same person. You will notice that short or one-word appositives such as in the phrase "my coworker Phyllis" do not have to be set off with commas. If you think a sentence or phrase would be clearer by setting off a short appositive with commas, then, by all means, do it. However, it is not absolutely necessary.
6. Use a comma to set off expressions of contrast.
7. Use a comma to set off a quote, dialogue or direct address.
8. Use a comma to signal that the main, independent clause is about to begin (after an introduction) or that it is being interrupted. Words or phrases that may be used to introduce or interrupt a sentence include: in fact, on the other hand, to tell the truth, yes, no, indeed, well, nevertheless, however, in my opinion.
9. Use commas in addresses, dates and large numbers.
10. Use commas in the salutation and closing expression of a letter.
If you're writing a formal business letter, you might substitute a colon (:) for the comma in the greeting.
The closing, however, will always use a comma.
If you can master these 10 comma rules, your writing will be neater, clearer and perfectly acceptable to English teachers and grammar purists everywhere, no matter where they stand on the gray areas.
To conclude your comma mastery learn how to avoid a comma splice in your sentence with these examples.