P1,+P2,+P3

Thesis - this is in your introduction and is a clear statement that expresses what you will argue in your essay. In this case, "The Constitution works because..." is your opinion followed by your "facts" that can be things like checks and balances, separation of powers, the amendment process, etc. The reason the first part is your opinion is because of one word - works. This is your opinion that the Constitution is a good constitution versus a bad one. The three main topics are facts because they in and of themselves are not arguing anything. They are supporting your argument. Things like checks and balances are facts because you can look it up to simply know what it is without it being positive or negative. However, a statement like "Checks and balances enhances the effectiveness of the Constitution." is an argument. This is fine for your body paragraphs but not your thesis. Avoid trying to prove your thesis in your introduction. That's what the body paragraphs are for.

Structuring your essay's paragraphs can help tremendously in making it easier to organizing your thoughts and write a better essay. P1, P2, and P3 is a way of organizing your essay's paragraphs in an outline before you actually start writing your paragraphs. Below is an example:

Body Paragraph 1 P1 - Topic sentence P2 - supporting detail for P1 P2 - supporting detail for P1 P2 - supporting detail for P1 P3 - example supporting P2s

Body Paragraph 2 P1 - Topic sentence P2 - supporting detail for P1 P2 - supporting detail for P1 P2 - supporting detail for P1 P3 - example supporting P2s

Body Paragraph 3 P1 - Topic sentence P2 - supporting detail for P1 P2 - supporting detail for P1 P2 - supporting detail for P1 P3 - example supporting P2s

One thing you can do is see if your paragraphs are organized like this to avoid writing a narrative or going off topic.