A topic sentence states the paragraph's main idea or focus. In multi-paragraph essays, the topic sentence signals the beginning of a new idea that differs from the previous paragraph's main idea. Most topic sentences start a paragraph, followed by supporting sentences, but occasionally they appear at the middle or end of the paragraph, to build climax or summarize key points.
Make It Clear
A strong topic sentence includes clear, specific language and avoids the use of vague, empty words. For example, "Blueberries are good for you" is a weak topic sentence that says very little, while "Blueberries provide essential nutrients" clarifies what makes blueberries good for you and lets the reader know that those essential nutrients will be explored in further detail. Since your topic sentence gives readers a preview of the information that follows, it needs to include a clear, controlling idea that the rest of your paper will support with evidence and examples. Failure to provide clarity in your topic sentence will make it harder for your reader to understand what should be expected and more challenging for you to write the rest of your paper.
Keep It Concise
Readers often find wordy topic sentences confusing and difficult to read, but a tightly written topic sentence succinctly conveys its message. For example, writing that "Boston is a cosmopolitan city" maintains reader attention more easily than "Boston is a city with people from many parts of the world," because it uses fewer words to express the same meaning.
Don't Be Boring
A compelling topic sentence captures the reader's attention and encourages him to read further. Including interesting or surprising facts within the topic sentence can spark readers' interest, as can unusual grammatical structure, such as framing the topic sentence into a rhetorical question. An interesting topic sentence is one that is well thought out and focuses on unusual or less common aspects of the topic rather than stating the obvious.
Within persuasive paragraphs, effective topic sentences strongly indicate the writer's position. For example, the delivery of "Individuals should consider decreasing their sugar consumption since it could lead to health problems" proves weak and sterile. Conversely, "Individuals should limit their sugar consumption to avoid health complications" comes across as more authoritative and confident in expressing the health risks associated with sugar consumption.
Use Active Voice
William Strunk Jr. and E.B. White explain in "The Elements of Style" that active voice, as opposed to passive voice, is more direct and forceful. For example, "Discount travel search engines are used by travelers as an efficient way to plan vacations" -- a topic sentence in passive voice -- has less impact than "Travelers use discount travel search engines to efficiently plan vacations," which is written in active voice. To check if your sentence uses active voice rather than passive, make sure that the subject of the sentence performs the action.