Key Concept Identification
Many students struggle with differentiating between the topic sentence and the main idea of an essay. An easy way to identify the difference is by focusing on the role each plays in developing the thesis of the essay. The main idea of a paragraph represents the specific point being made in the paragraph. It is a summary of what the argument is trying to argue. A topic sentence explicitly defines what the paragraph will argue, in general terms. For example, if the main idea of the paragraph is that cell phone use has resulted in a decline in spelling ability, a topic sentence would state that argument in clear language: "Cell phone use among children is causing a general decline in spelling ability."
A topic sentence is a visual representation of the main idea of the paragraph. Subsequent sentences will relate to the main idea of the paragraph by providing support and evidence that backs up the topic sentence. Thus, a reader is told what will be argued through the topic sentence, while the main idea is constructed through the supporting details offered in the paragraph.
A main idea may be very broad in nature. Furthermore, it is often not confined to a single paragraph, as it may represent the overall theme of the essay. Therefore, a topic sentence is used to focus the paragraph, to unite it to a specific component of the main idea of the essay and thus develops the main idea of the paragraph. The topic sentence restricts the information that is to be presented in a paragraph. For example, if you argue that "auto correct removes the need to learn how to spell," the topic sentence would limit your paragraph to this concept and would dictate that any information that does not directly relate to the topic sentence would be contained in a different paragraph.
The topic sentence is part of a greater road map for an essay. It relates to the thesis statement, which presents the overall argument of the essay or the essay's general theme. Each topic sentence represents a new dynamic of the argument, signifies a new paragraph and develops internal consistency. Each topic sentence illustrates the author's points and if a reader was to present a summary of the writer's argument, he or she could use the topic sentences to outline each component. Due to its importance in the essay's organizational structure, topic sentences are generally located at the beginning of the paragraph to set the stage for the details that follow.