Commas have several purposes. One is to protect sentences. They separate out what is not necessary to the sentence itself. Often that is extra information needed by the reader but not by the sentence.
Think of a sentence as an interstate highway, allowing only limited exits. To travel from Teaneck, New Jersey, to downtown San Francisco, California, you stay on the road. If you wish to stop for gas or for food (or to sleep), you need to take some exits. Those exits are necessary for the driver and the car but not for the highway.
Information between commas is necessary for the reader, like stopping for gas. The sentence itself does not need it. Look at the sentences in these paragraphs. The phrases in italics are taking exits. Each sentence is complete without the italicized words.
With that in mind, try these five sentences. Where do the commas belong? What information is helpful for the reader but not needed by the sentence?
1. Momma I don’t like lettuce.
2. Before we go I will ask Thomas when the game begins.
3. George Washington our first president was born in Popes Creek Virginia.
4. When he was a child tradition says he chopped down a cherry tree.
5. “I cannot Father tell a lie.”
1. Momma, he hit me.
Who the child is talking to (“Momma”) is not necessary for the sentence (“he hit me”).
2. Before Sunday, I will ask Thomas when the game begins.
When I ask Thomas is not important to the sentence (“I will ask Thomas when the game begins”).
3. George Washington, our first president, was born in Popes Creek, Virginia.
“Our first president” adds information about who George Washington was.
“Virginia” tells the reader where Popes Creek is.
The sentence (“George Washington was born in Popes Creek”) does not need either.
4. When he was a child, tradition says he chopped down a cherry tree.
“When he was a child” tells us when this happened. The sentence itself does not care.
(No comma after “says” because these are not the exact words tradition would say.)
5. “I cannot, Father, tell a lie.”
That the child is speaking to his father does not matter to the sentence itself.