Thesis statements and topic sentences are your guide to writing your essay, and they are also a guide for the reader in reading your essay. They both have the same essential purpose, but have different placements in an essay, so we will examine them one at a time.
When you write an essay, you have some main idea you want the reader to take away after reading your essay. We call that your thesis. In a persuasive or argumentative essay, your thesis might be that we all need to start doing something about global warming and climate change. In your thesis statement, you need to let the reader know that, but you also need to let the reader know how you are going to support your main idea and what points you will discuss about it. Those points for an essay on global warming and climate change could be that rising sea levels will destroy many places on earth, extreme weather events will cause more and greater death and destruction, and food sources will be severely compromised. Here is a thesis statement I could write:
It is vital that we all pitch in and stop or slow down global warming and climate change because the rising seas will destroy occupied land, more extreme weather will harm more people and property, and we will not be able to raise enough animals and grow enough plants to feed the earth.
This thesis statement would be at the end of my introduction, where it acts as a kind of table of contents for the reader, a guide to the entire essay. You are also going to use it as your guide to write your body paragraphs, which is where the topic sentences come in.
Using my hypothetical example, I will write three body paragraphs in my essay, one for each of the points named in my thesis statement, and these will be in the same order as the points in my thesis statement. My first body paragraph will be about the rising sea levels. My second will be about the extreme weather, and my third will be about animals and plants.
The first sentence I write for each of these body paragraphs is a topic sentence, which is a sentence that lets the reader know what point I am going to discuss to support my thesis. This acts as my guide, too, as I write. Having a topic sentence keeps me on track so I do not write about animals and plants in my paragraph about rising sea levels. Everything I put in the body paragraph will be focused just on that topic, that point. Here is a topic sentence I could use for my first body paragraph:
If we do nothing to stop the warming of the earth, the seas will rise enough to place islands underwater and to overrun coastal areas.
Now I can stay focused on providing evidence to support that, and the reader will understand right away what point I am making.
Once I have written my three body paragraphs, I will write one more paragraph, a conclusion, and in that paragraph, I will remind the reader what the thesis is and what the supporting points are, but not in the same exact words as I used in my thesis statement.
You can see how important it is to have a thesis statement and topic sentences when you write. It keeps your writing focused, and it guides the reader beautifully, increasing the chances your writing will be remembered and prompt action.