In the story "The Tell-Tale Heart," the speaker is very concerned with letting the reader know that he is not "mad" (crazy), as he thinks that if the reader believes him to be insane, he will not believe the story he is telling, or will judge his actions differently.
However, the speaker is actually mad, and this is shown through certain symptoms and behaviours the speaker exhibits. In an effort to hide these, the speaker explains them by saying that he suffers from "nervousness," but he refuses to admit that his actions are actually driven by insanity.