What is a topic sentence and what's a good example of one?
what is a topic sentence?
While I agree that a topic sentence introduces what a paragraph will discuss, it is not a "thesis" in any sense. Your thesis is your main argument. Each body paragraph then presents evidence and argumentation that supports the thesis statement. In a five-paragraph essay, there is the thesis paragraph, three body paragraphs--each of which presents one piece of evidence (for a total of three)--and a concluding paragraph. The topic sentence presents the evidence; that is why it's the topic of the paragraph. The remainder of the body paragraph interprets the evidence and explains how this helps to prove the thesis.
The answer to this question depends on which part of the essay you are working for. I will use the story "Young Goodman Brown" and the idea that the story's setting is important our understanding of its purpose as an example.
Introduction - The topic sentence of an introductory paragraph is usually very broad that connects to your topic. It is something that everyone will agree with.
Example: Sometimes the place where a story occurs can play an important role in how the characters function throughout the events taking place.
Body - The topic sentence for a body paragraph is very important. It tells your reader EXACTLY what you will be approaching in that paragraph.
Example: It is clear that the forest, where Young Goodman Brown's journey takes place, along with the characters he encounters, plays a significant role in his experience, as it is the place where all of his beliefs are questioned.
Conclusion - The topic sentence in your conclusion is very specific and captivates or summarizes your overall argument.
Example: As you can see, the Puritanical world where Young Goodman Brown comes from, along with the forest where he takes his journey, define who he is and what he becomes at the end of the story.
There seems to be a debate brewing here about whether a topic sentence is a type of thesis. To add clarity, I'll point out some connections and comparisons between the two types of statements.
It's possible to compare them on the basis that they both contain main ideas that give structure to the essay; however, as some have pointed out here, the thesis statement and topic sentence do not serve the same purpose. Consider that the topic sentence should NOT ONLY introduce the topic and main idea of the paragraph BUT ALSO support some aspect of the thesis, so it could be considered a simple paragraph summary that bridges some thesis ideas to the paragraph ideas. Conversely, it's also necessary that the thesis statement contains at least a hint about each of the topic sentence main ideas.
One common mistake is to confuse main ideas with examples or evidence, which should not be emphasized (and usually should not even appear) in a topic sentence or thesis. Generally, they should appear later in the body paragraphs as support. An example is used to show (or illustrate) the topic idea in a practical context. Evidence is used to establish (or prove) the validity of the topic sentence. Either way, these supporting ideas are not the main idea of the paragraph, and, therefore, should not be the main point of a topic sentence. (For the same reason, this principle is also true of the thesis statement.)
A topic sentence should introduce what a paragraph is going to talk about. Like Jayanthi said, it should serve as the "thesis" of the paragraph. The important thing about this is that your topic sentence should include a statement that a reasonable person can argue with, not just a factual statement.
Example ofan ineffective topic sentence: World War II cost millions of lives on several continents.
Exaple of an effective topic sentence: Because of World War II's astronomical body count, the United States should have deployed forces while the war was still in its earlier stages.
A topic sentence should let your reader know what you're trying to persuade them to think. I hope this helped!