These worksheets gives more or less complete explanation how to define the main idea of the paragraph or a short passage. As a rule main idea materializes in a topic sentence the rest part of the paragraph or a short passage provides supporting details: arguments, examples, numbers and so on. It seems that it is more supportive to imagine the structure of the passage in graphics. The triangle diagram shows the relationship between the ideas in the passage. When the main idea is the first sentence all the other support it, If the topic sentence is in the last sentence the main idea conclude the entire specific details. As for the main idea within the paragraph usually it is the second sentence while the first one is an introductory and explanation follows after it.There are some examples with a detailed explanation. I hope it is helpful for students in order to understand the text better and useful in summarizing it.