An introductory phrase is a group of words at the beginning of a sentence. This is often a prepositional phrase telling more about when or where something is happening.
In the morning, I walk my dog.
After breakfast, we do the laundry.
Along the way to school, I saw some baby squirrels.
Under the sofa, my dog whimpered and howled.
We also use a comma after an introductory participial phrase. This type of introductory phrase modifies the noun that comes directly after.
Running quickly, Susan escaped the dog. Running quickly describes Susan.
Opening the door, Mother invited me inside. Opening the door describes Mother.
Sniffling and crying, Betty launched herself into my arms. Sniffling and crying describes Betty.
Place a comma after an introductory phrase at the start of a sentence.
▶Now you try it. Place a comma into this sentence: In the beginning we have much to do. Did you put the comma after the introductory phrase? Good for you!