When you first begin teaching your third-grade students about identifying the main idea of a story, lead them through the process, starting with smaller passages. Read the passages together as a class, allowing student volunteers to read aloud. After each passage is read, ask the class what they believe is the main idea of the story. For each answer, ask the child who answered to elaborate why he feels that is the main idea. Some stories may have more than one right answer.
Once your students have a firm grasp on what a main idea is and how to identify it, you can allow them to practice on their own. An idea web consists of a larger circle that branches out into smaller circles. Students read a specified passage or short story. After reading the story, the students write the main idea of the story into the larger circle. The students must identify the supporting ideas that contribute to the main idea by placing them into the smaller circles surrounding the larger one. You may also want to use an art project to create an idea web, such as tracing the children's hands and using the palm for the main idea and the fingers for supporting ideas.
Children often learn well through repetition. The more often they practice a skill, the better they become. This also applies to identifying a main idea and reading comprehension. Create a worksheet or quiz for your class. Use several short reading passages to construct the papers. Beneath each passage, create multiple choice answers, one of which is the correct answer and several other somewhat feasible answers. You can also provide a blank line if you think your class is capable of determining the main idea on their own.
Instead of requiring your students to identify a main idea in a story written by someone else, you can help them learn about main ideas through their own writing. Choose a main idea or a list of main ideas from which your class can choose. Ask your students to write a short story or a paragraph that covers the main idea. By working through the process of writing based on a main idea, your students will learn how writers implement a main idea into their stories.