To refresh your memory, a clause has a subject and a verb. A phrase is a group of words with no verb and no action.
If you begin a sentence with an introductory phrase, you must use a comma after the phrase. Look at these examples:
In the morning, I like to sleep late.
Last night, we had a big storm.
In the beginning, it was raining gently.
After awhile, the wind got strong and hail blew against the house.
Each of these introductory phrases is a group of words with no action inside. Each introductory phrase has a comma after it.
Sometimes, you begin a sentence with just an introductory word. These introductory words are called interjections. Look at these examples. Notice the comma after each introductory word.
Wow, that’s a big fish!
No, I don’t want to go shopping right now.
Yes, I did the laundry.
Gee, that’s a pretty dress.
Use a comma after an introductory phrase and after an introductory word.