You will gradually absorb a sense of when rules can be broken. Punctuation, when understood with sensitivity, leads to riffles and pools of narrative flow. Yes, I'm deliberately playing on the theme of streams, because we all know that streams flow irregularly. The fast and slow reaches are part of their fascination. In writing, riffles are forceful points of the narrative and longsentence runs.
Pools are short sentences.
Pools are breaks in the flow of words, sometimes providing a welcome interruption in a spate of ideas. Clarification results as ideas settle. I have learned a lot by reading fiction. I read books for relaxation and for the story, although I will admit that I am rarely able to ignore structural style, and some authors make me itch to edit. Indeed, I wonder how some socalled editors in the novelpublishing business were ever hired.