Final answer:
The first sentence in a paragraph that states the main idea is called the topic sentence. It differs from the thesis statement, which is the main argument for the entire essay usually placed at the end of the introduction.
Step-by-step explanation:
The first sentence in a paragraph that announces the main idea is called the topic sentence. This sentence is crucial as it summarizes the main argument or point of the paragraph and provides a clear direction for what the rest of the paragraph will discuss. Unlike the thesis statement, which articulates the main argument of the entire essay and is typically found at the end of the introduction, topic sentences occur at the beginning of each body paragraph. They are major supporting statements for the thesis, and every topic sentence should align with and support the thesis statement. To effectively plan the main points and body paragraphs, your main points should be your topic sentences, expressed as one sentence at the beginning of each paragraph, previewing the main idea because they tie back to the thesis statement.