When using introductory words or phrases, you want to use a comma to separate them from the rest of the sentence:
- When we are cooking children cannot come in the kitchen.
- When we are cooking, children cannot come in the kitchen.
The first sentence might lead your reader to suspect you of cooking children; however, using a comma after your introductory phrase avoids confusion and clarifies your meaning.
Also, we must use a comma when addressing someone by name: this is referred to as the direct address comma. When addressing someone directly, you need a comma before the name if it's at the end of the sentence:
- Let's eat Grandma!
- Let's eat, Grandma!
The sentence without the comma tells the reader you want to eat Grandma, instead of telling Grandma that it's time to eat. Again, without the comma, the sentence may lead the reader to accuse you of cannibalism!
The direct address comma is necessary when the name is at the beginning of the sentence as well. Place the comma after the name to clarify your meaning. For example:
- George Harrison has been sort of my hero.
- George, Harrison has been sort of my hero.
Also be sure to use two commas to off-set the name if it's in the middle of the sentence:
- Let's eat Grandma and not wait for the rest of the family.
- Let's eat, Grandma, and not wait for the rest of the family.
Yes, commas are overused and misused in the English language: as Oscar Wilde said, "I have spent most of the day putting in a comma and the rest of the day taking it out." However, we must remember that correctly utilizing the comma saves lives and keeps us from "cooking children" and "eating Grandma."