We sometimes begin a sentence with a prepositional phrase. This is called an introductory phrase. Always use a comma after an introductory phrase. Here are some sentence examples:
After breakfast, Meg did the laundry.
Before dinner, Sam practiced his piano piece.
At the end of the street, two men are fixing a broken car.
In the kitchen, Mom and my little brother are decorating cookies.
Notice that you can change the order of words in each of these sentences. If you move the prepositional phrase to the end of the sentence, you do not need the comma.
Meg did the laundry after breakfast.
Sam practiced his piano piece before dinner.
Two men are fixing a broken car at the end of the street.
Mom and my little brother are decorating cookies in the kitchen.
If you begin a sentence with a prepositional phrase, you must use a comma after the phrase.
▶Now you try it. Look at this sentence. Does it need a comma? In the basement my friends are playing ping pong. I hope you put a comma after the introductory phrase.