* Nonfiction: The main idea in nonfiction is a factual claim or argument supported by evidence. The author's goal is to inform, explain, persuade, or describe something real, based on verifiable information. The main idea might be a central thesis, a key concept, or a significant finding. The truth or accuracy of the main idea is paramount.
* Fiction: The main idea in fiction is a central theme or message explored through narrative. The author's goal is to create a compelling story, engage the reader emotionally, and potentially explore universal truths or human experiences. The main idea is often implicit, revealed through plot, character development, and setting rather than explicitly stated. While a fictional work might be inspired by reality, the truth or accuracy of the events depicted isn't the primary concern.
In short: Nonfiction focuses on presenting a factual main idea, while fiction focuses on exploring a thematic main idea. One emphasizes accuracy and evidence; the other emphasizes narrative and interpretation.