Whenever I'm writing, I use this style:
"I don't see why I have to put up with this nonsense." I muttered.
However, I see this type of writing in books:
"This room is full of idiots," he muttered.
Is the style I use okay, or do I have to change it to the one I see in books?
If you end the quote with a period, that makes it a complete sentence. And that turns:
I muttered.
into a second sentence.
If you want to indicate that he (or I) muttered the words between the quotation marks, then yes, you must end the quoted words with a comma.
Dialogue has a bit of a different style. Here's some examples of correct dialogue grammar (with respect to punctuation at the end):
"You're looking well this morning," said Tommy.
"I'm doing well," said Theresa, "thank you kindly."
Katherine said, "Let's have pasta for dinner."
"That sounds good," said Austin. "Do you want to go to the place on Broad Street?"
So yes, the way you see dialogue written in books is correct and you should change the grammar of your dialogue to match.
I agree with the two other posters and would like to caution you from always using correct grammar for dialogue. When people are talking out loud, they don't use it.
Off the top of my head, this is an example:
"Something I said?" he asked.
"Every time, another complaint," she said.
"Try living with yourself sometime," he said.
"Comm Ave," she said. "There."
"What?" he asked.
"Comm Ave, where I lived with myself and loved it. Absolutely."
If you're at a restaurant or coffee shop, try recording two people nearby talking to each other. Just for a brief bit. Then take dictation - write down exactly what each one says to the other from the recording. It will inform your writing.
