1) Paragraph unity means that each paragraph focuses on only one main idea or topic, as expressed in the topic sentence. All other sentences and details in the paragraph must relate back to and support this single main idea.
2) Unity is important because it helps readers easily follow the writer's ideas. When a new paragraph begins, readers know it will be about a new topic.
3) Some rules for writing unified paragraphs include having a clear topic sentence, using evidence and details that illustrate the topic sentence, and explaining how the evidence relates back to the main idea. Paragraphs should also be logically related within longer essays or documents.