How to write a thesis sentence.

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The thesis sentence is the soul of the document.  It is the message you want the reader to remember, your purpose for doing this.

Express your truth, the truth you choose. The thesis statement is the core of the essay, it's essence condensed into a sentence.

One Composition = One Big Idea
The thesis sentence is the sentence that expresses the big idea.
(A great place for the thesis sentence is at the end of the first paragraph)

If you are writing in the form of paragraphs, it is good to put the thesis at the end of the first paragraph, because that is where the reader's attention will linger for a moment before moving on to paragraph two.

The only other thing you need to know: The first sentence of each "body" paragraph of the essay expresses the main idea of its paragraph. Each paragraph has a main idea that is expressed in that first sentence, so it is called a "topic sentence."

Here is an easy way to understand good "structure" for your document:

One Composition = One Big Idea
The thesis sentence is the sentence that expresses the big idea.
(A great place for the thesis sentence is at the end of the first paragraph)
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One Paragraph = One Smaller Idea 
It expresses a smaller idea that helps to explain or verify the thesis statement.
(Each paragraph begins with a "topic sentence" that expresses that idea.) 

Here is the secret to writing a good thesis sentence: Let it become "sharper" (more specific and unique) while you write the essay.  Keep on tweaking the thesis while you write the body paragraphs.

Usually, it is best to write the thesis statement AFTER you have written some of the body paragraphs, because that is the only way you will know what you are talking about.  Each body paragraph starts with a topic sentence that expresses the paragraph's main idea, and...

The thesis sentence at the end of the first paragraph will say something that encompasses all the supporting ideas expressed in those paragraph topic sentences.