Main Idea
When looking for the main idea of a paragraph, look for a common theme connecting the sentences, and if the first and last sentences are asking or answering anything.
In an essay or research paper, the main idea is contained in your thesis statement. A thesis is usually the last sentence of an introduction paragraph. The body paragraphs will provide supporting evidence / supporting close readings for the thesis, with the last sentence of each paragraph summarizing the connection between the evidence/reading and the thesis.
In a narrative text (i.e., a passage in a story) pay attention to any definitive statements about a character's opinion, demeanor, emotional state, intentions, and actions, or a dynamic or interaction between characters, or the setting. Definitive descriptive statements will reveal the author's main idea. Similar to an essay, looking for connections between the first and last sentences, or identifying a repetition of ideas, is helpful.
Inference
Paying attention to descriptive elements about a character or their setting leads to an inference about the character or the plot. Unlike a main idea, an inference is not explicitly stated, rather, it emerges through a pattern of connected descriptions or ideas in a text. By identifying connections between descriptive sentences or general themes, you can infer a larger point or stance the author is trying to make.
In an essay, you should also look at common themes and the first and last sentences. Unlike deciphering the main idea, deciphering an inference involves attending to connections between key sentences, such as the topic and explanatory sentences, and thinking about a point or stance that is supported by that connection but is not explicitly said. You should also think critically about the argument or analysis presented, and if you find yourself thinking about a larger idea. Remember: an inference is an idea or argument supported by the essay's analysis and evidence but not explicitly stated.