A topic sentence introduces the controlling idea of a paragraph and signals its relationship to the essay's thesis. Paragraph unity means every sentence in the paragraph develops that single controlling idea and nothing else. When a paragraph drifts into a second idea, it either needs to be split or refocused. Strong topic sentences are both specific enough to govern the paragraph and general enough to require the sentences that follow.
Practice the 'paragraph audit': read only the topic sentences of a multi-paragraph essay to verify they form a coherent outline. Then check each body paragraph by deleting every sentence that could exist in a different paragraph — anything that survives is extraneous.
From your work on paragraph structure, you know what a paragraph is: a unit of thought organized around a single controlling idea. The topic sentence makes that idea explicit and governs everything that follows. Think of it as a contract with your reader — the topic sentence promises a certain territory, and every subsequent sentence in the paragraph is bound by that promise.
A topic sentence must do two things simultaneously: be specific enough to actually govern the paragraph, and be general enough to require the sentences that follow. "Shakespeare is a great writer" fails the specificity test — no single paragraph could develop such a large claim. "Shakespeare's use of iambic pentameter in *Hamlet* creates a tension between formal control and emotional instability" is specific enough to govern a paragraph and general enough that the reader expects supporting evidence. Think of the topic sentence as a mini-thesis: just as the essay's thesis promises what the whole argument will demonstrate, the topic sentence promises what this one paragraph will demonstrate. The body sentences are the proof; the topic sentence states the claim.
Paragraph unity is the discipline of holding to that promise. Every sentence in a unified paragraph should answer the question: "Does this develop the controlling idea stated in the topic sentence?" A sentence that introduces a new claim, opens a digression, or makes a point without tying back to the topic sentence breaks unity. When you find yourself writing a sentence that doesn't fit, it usually signals that you've discovered a second idea deserving its own paragraph — or that your topic sentence is too narrow for what you actually want to argue and needs revision.
The paragraph audit is the most useful diagnostic tool: read only the topic sentences of a multi-paragraph essay, in order. If those sentences alone form a coherent, sequential argument — each advancing the essay's thesis in a distinct direction — the essay is structurally sound. If the topic sentences could be reordered without confusion, the essay lacks logical progression: the paragraphs are associated by topic rather than linked by argument. Running this audit on your own drafts before revision quickly surfaces structural problems that line-level editing will not fix. Applied to texts you read, the same technique reveals whether a writer is actually making an argument or merely organizing information by category.