A theme is a central topic, subject, or message within a narrative. Themes can be divided into two categories: a work's thematic concept is what readers "think the work is about" and its thematic statement being "what the work says about the subject".
The main idea is the point of the paragraph. It is the most important thought about the topic. The author can locate the main idea in different places within a paragraph. The main idea is usually a sentence, and it is usually the first sentence. The writer then uses the rest of the paragraph to support the main idea.
