Acomma may be used correctly to do any of the following (a) join sentence when used with a conjunction. (b). set off an introductory word, clause, or phrase. (c) set off interrupting words or expression. (d) call attention to what follow it

(d) call attention to what follow it

A comma may be used correctly to do any of the following:
(a) join sentence when used with a conjunction.
(b). set off an introductory word, clause, or phrase.
(c) set off interrupting words or expression

Examples:

When he came, his mother ran unto him.
Talking about bugs, I don't like any of them.
Hey, did you do the thing I told you to do?
if you don't know how to answer that, you can ask me for help.