A main idea usually has two parts. The subject is the thing or idea you are talking about, like "a main idea." The predicate is what you are saying about the subject, such as that it "usually has two parts." When looking for a main idea, try to find what the writer is talking about and what she is saying about that subject. When creating one, decide what you are writing about and then what about it you want to explain or convince readers of.
While a longer piece of writing like an essay or article usually has a main idea, each paragraph in that piece may have its own smaller main idea, or sub-point. Each sub-point should support the main idea of the overall piece of writing, and the sentences in each paragraph should support the sub-point that the paragraph is making. The main idea of a paragraph is often stated in a sentence called the thesis statement, which is typically (but not always) found at the beginning of the paragraph.
Essays, research papers and persuasive writing should always be built around one idea. You can strengthen your main idea by explaining it in more detail, providing facts that support what you are saying or using arguments to try to change your reader's mind. To make your writing clearer and stronger, go through what you have written and check each sentence to make sure that it helps the reader understand, believe or care more about your main idea.
Understanding a writer's main idea will help you understand what he writes and why. The writer's main idea is the reason he wrote the piece you are reading. You can often find the main idea at the beginning or end of a piece of writing, and every sentence in the piece should help you understand or agree with the main idea. One way to find the main idea is to see what most of the sentences are talking about, then reread the piece to see what the writer says about that subject.