The central idea is important to the entirety of the essay, a main idea involves most of the essay but isn't necessarily important to all parts.
eg.
Central idea: Gravity follows an inverse square law dependent on distance.
Main idea: John Keats had four daughters - each of whom we will consider briefly and separately.
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There is not necessarily any difference between the main idea and the message of the author. The message of the author would have to be somehow connected to the main idea, because the purpose of the author is to put forth diferences in certain ideas as contrasted with the author's feelings about them.
These two terms are pretty much synonymous however there is a slight difference in connotation.
A main idea might be more specific than a theme. For example, the theme might be food, and the main idea might be that people need to be more careful about what they eat. I am sure there are also many informational texts in which there is no difference between the theme and the main idea.
The main idea is the central point or topic of a text, while the author's message is the underlying theme or lesson the author wants to convey to the reader. The main idea is what the text is about, while the author's message is the deeper meaning or purpose behind the text.
The theme is just the kind of idea behind the story. The main idea is the idea hat will determine what events take place and how they occur in the story.