What is the difference between a stated main idea and an implied main idea?
The difference is that the stated main idea is there in the text but the implied main idea is what you think the author was trying to convey.
I'm not sure that a clear distinction is universally recognized, although you could certainly try to draw one. Both implied and inherent powers are as opposed to explicit powers. An explicit power is one that is enumerated in the Constitution or an Act of Congress. If you wanted to distinguish implied and inherent power, implied powers would be ones that are not enumerated, but follow logically from ones that are enumerated. Inherent powers would then…
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.