Full answer
A good essay rests entirely on its thesis — a single, arguable claim that the rest of the paper exists to prove. Not a fact ("World War I began in 1914") and not a vague observation ("war is complicated"), but a specific, defensible position that a reasonable reader could disagree with. Everything in the essay should either support this claim or acknowledge and rebut the strongest counterargument.
The introduction should funnel from broad context to specific thesis in three to four sentences. Avoid starting with a dictionary definition or a sweeping statement like "Since the dawn of time..." — these are filler. End the introduction with the thesis as the final sentence, where it anchors the rest of the paper.
Each body paragraph follows a predictable structure that helps both writer and reader: a topic sentence stating the paragraph's claim, two to three pieces of evidence or examples, and analytical sentences that explain exactly how the evidence supports the thesis. The evidence does not speak for itself — analysis is the work that earns the grade.
The conclusion should not restate the introduction. Instead, it should articulate why the argument matters — what the implications are, what question it opens up, or what action it suggests. A conclusion that simply repeats the intro signals that the writer has run out of things to say. A forward-looking final sentence leaves the reader with a reason to keep thinking.