For a reader, processing dialogue in a story can be tricky enough as it is. The reader has to keep track of exactly who is talking and what tone of voice each person is talking in. The author needs to make sure it is always crystal clear who is talking and how they are saying the words.
Punctuation is a key part of that.
I’ll mention that these examples all us US English, where we use double quotes around dialogue. In British English, they use single quotes.
The first rule of punctuation is that the punctuation for a line of dialogue always goes INSIDE the quotation marks. So you have examples such as:
Mary yelled, “Stop doing that at once!”
Sally asked, “What do you think you are doing?”
Carlos said, “It’s time for us to go.”
The second rule is that you ONLY join a surrounding phrase (like Carlos said or Mary yelled) to the dialogue with a comma if that surrounding phrase specifically describes how the words themselves were said. You do NOT join the phrase with a comma if it’s just describing something else the person is doing.
So, this is correct:
Bob whispered, “I love you very much.”
This is NOT correct:
Lisa bit her lip, “I love you, too.”
It would need to be:
Lisa bit her lip. “I love you, too.”
The action is SEPARATE from the dialogue being spoken. It doesn’t describe how the words are being said.
So this is correct:
James screamed, “I will conquer you all!”
This is NOT correct:
Oliver kicked at the rock, “You can just try to win.”
It would need to be:
Oliver kicked at the rock. “You can just try to win.”
Ask with any questions!