Read the paragraph and highlight any words or phrases that the author uses that seem important to the overall paragraph. If, for example, he writes something like "The United States had to enter into the Vietnam war to try to end the spread of communism," you'd want to highlight the words "had to enter into the Vietnam war" and "end the spread of communism."
Identify the three main parts of the paragraph. These comprise the topic sentence, the body sentence, or sentences and the concluding sentence. Typically the topic sentence introduces the subject of the paragraph, the body sentences expound on the subject, and the concluding sentence delivers the main idea of the paragraph.
Write a sentence that summarizes the paragraph. Use your highlighted words to help you in doing so. Once you have finished writing the sentence, see how it compares with the paragraph. If it summarizes the paragraph in the most succinct way, you now have your main idea.
Read the short story as many times as you like and write down any prominent characteristics of the story that you find revealing about its main idea. Note any unique character traits, plot points, settings, dialogue and descriptions that might be important.
Write a summarizing paragraph that details the story, including what the characters go through. Break down the story into three parts: the beginning, the middle and the end, or the introduction, the climax and the conclusion. Use succinct language and include only the most vital parts of the story.
Read your summary and ask yourself: "What was the author trying to show his readers?" or "What's his point in writing?" If, for instance, he wrote about a girl who slips into a fantasy world when her father is dying, you could say that the main idea of the story is that the girl was trying to cope with her father's death by breaking away from reality. There is always a theme or main idea in any story; you just have to ask yourself questions about the story to find it.