A main idea is what the book is mostly about. In a main idea you explain the main point of what the author is explaining to you(Like short summary). A theme is a message, lesson, or moral of the book.
The clause <em>who bought the coat </em>is used as an adjective subordinate clause because it modifies the noun <em>the man. </em>What man forgot his umbrella? (The one) who bought the coat.<em> </em>
