A comma can make or break a sentence.

Let’s eat Mama!

Let’s eat, Mama!

In both sentences, the speaker is clearly hungry – in the first one, to eat “Mama,” which is wrong on about a million different levels.

Without the comma, Mama is a direct object, which means it receives the action, which, here is “eat.”
In the second sentence, however, the comma clears up this confusion, and it becomes apparent that they are addressing (or talking to) Mama, not planning on eating her.

Please notice that if you were to read this out loud, there would be no real difference between the two. A comma does not always denote a pause as many assume (or have been taught).

Here are some common – but not all of the – reasons you’ll need a comma:

1. A series of three or more.

I had kiwi, pineapple, and muss melon in my fruit salad.

There’s no doubt that the comma after kiwi belongs, but many would omit the one after pineapple. While it is not necessarily wrong to omit this comma (called a serial or Oxford comma), it can cause confusion to do so.

2. Joining two sentences with “and,” “but,” or “or.”

I love writing this blog, but tonight, I almost forgot about it!

On either side of the word “but” is a full sentence – subject, verb, and object.

I went to the store and I bought some ice cream.

Omitting the comma before “and” creates a fused sentence, sometimes also called a run-on sentence. Some others (creative writing) will leave out the comma to create a certain effect.

3. Showing that multiple adjectives are describing the same noun.

I love my cozy, yellow sweater.

The comma between “cozy” and “yellow” shows that both words are describing the sweater: it is both cozy and yellow.

I love my cozy yellow sweater.

Although most people would read this sentence the same way, it could also be interpreted to read the sweater is a cozy shade of yellow.

4. Setting off introductory phrases.

When I went to the store last night, I had the hardest time finding almonds.

This sentence could also be written “I had the hardest time finding almonds when I went to the store last night.” Notice that no comma is used when the sentence is inverted. When the dependent clause comes at the beginning of a sentence, you follow it with a comma.

5. Setting off names.

Always, always, always use commas to set off the name of someone you’re speaking to from the rest of the sentence. This little piece of punctuation can make all the difference, like it does for Mama in our opening example.

Need more help with the comma (or other grammatical) issues? Check out the Purdue OWL (Online Writing Lab): http://owl.english.purdue.edu/owl/