You need a comma when directly addressing someone!
- Mom, have you seen my homework folder?
- Okay, Mario, let’s go.
- If I were you, Trina, I’d read this series by Lois Lowry.
You need a comma after an interjection or weak exclamation.
- Hey, Mom, what’s up?
- Sure, I’ll go along.
- Okay, I’ll take two.
- Yes, I’m talking to you.
- Well, what did he say?
- Wow, that was close.
- Hi, Mr. Williamson.
After a phrase or clause that starts a sentence, you need a comma.
- By the time I got to it, the ice cream cake had been devoured.
- When everyone was finally ready, we piled into my brother’s minivan.
- While I did my homework, Dad made breakfast.