语言教学 | 哈佛大学写作教学系列011-主题句与信号词

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来源：哈佛大学写作中心

征稿：二语写作

Topic sentences and signposts make an essay's claims clear to a reader. Good essays contain both. Topic sentences  reveal the main point of a paragraph. They show the relationship of each paragraph to the essay's thesis, telegraph the point of a paragraph, and tell your reader what to expect in the paragraph that follows. Topic sentences also establish their relevance right away, making clear why the points they're making are important to the essay's main ideas. They argue rather than report. Signposts, as their name suggests, prepare the reader for a change in the argument's direction. They show how far the essay's argument has progressed vis-à-vis the claims of the thesis. 

Topic sentences and signposts occupy a middle ground in the writing process. They are neither the first thing a writer needs to address (thesis and the broad strokes of an essay's structure are); nor are they the last (that's when you attend to sentence-level editing and polishing). Topic sentences and signposts deliver an essay's structure and meaning to a reader, so they are useful diagnostic tools to the writer—they let you know if your thesis is arguable—and essential guides to the reader

Forms of Topic Sentences

 Sometimes topic sentences are actually two or even three sentences long. If the first makes a claim, the second might reflect on that claim, explaining it further. Think of these sentences as asking and answering two critical questions: How does the phenomenon you're discussing operate? Why does it operate as it does?

There's no set formula for writing a topic sentence. Rather, you should work to vary the form your topic sentences take. Repeated too often, any method grows wearisome. Here are a few approaches.

Complex sentences.  Topic sentences at the beginning of a paragraph frequently combine with a transition from the previous paragraph. This might be done by writing a sentence that contains both subordinate and independent clauses, as in the example below.

 Although Young Woman with a Water Pitcher depicts an unknown, middle-class woman at an ordinary task, the image is more than "realistic"; the painter [Vermeer] has imposed his own order upon it to strengthen it. 

This sentence employs a useful principle of transitions: always move from old to new information.  The subordinate clause (from "although" to "task") recaps information from previous paragraphs; the independent clauses (starting with "the image" and "the painter") introduce the new information—a claim about how the image works ("more than Ôrealistic'") and why it works as it does (Vermeer "strengthens" the image by "imposing order"). 

Questions.  Questions, sometimes in pairs, also make good topic sentences (and signposts).  Consider the following: "Does the promise of stability justify this unchanging hierarchy?" We may fairly assume that the paragraph or section that follows will answer the question. Questions are by definition a form of inquiry, and thus demand an answer. Good essays strive for this forward momentum.

Bridge sentences.  Like questions, "bridge sentences" (the term is John Trimble's) make an excellent substitute for more formal topic sentences. Bridge sentences indicate both what came before and what comes next (they "bridge" paragraphs) without the formal trappings of multiple clauses: "But there is a clue to this puzzle." 

Pivots.  Topic sentences don't always appear at the beginning of a paragraph. When they come in the middle, they indicate that the paragraph will change direction, or "pivot." This strategy is particularly useful for dealing with counter-evidence: a paragraph starts out conceding a point or stating a fact ("Psychologist Sharon Hymer uses the term Ônarcissistic friendship' to describe the early stage of a friendship like the one between Celie and Shug"); after following up on this initial statement with evidence, it then reverses direction and establishes a claim ("Yet ... this narcissistic stage of Celie and Shug's relationship is merely a transitory one. Hymer herself concedes . . . "). The pivot always needs a signal, a word like "but," "yet," or "however," or a longer phrase or sentence that indicates an about-face. It often needs more than one sentence to make its point.

Signposts

Signposts operate as topic sentences for whole sections in an essay. (In longer essays, sections often contain more than a single paragraph.) They inform a reader that the essay is taking a turn in its argument: delving into a related topic such as a counter-argument, stepping up its claims with a complication, or pausing to give essential historical or scholarly background. Because they reveal the architecture of the essay itself, signposts remind readers of what the essay's stakes are: what it's about, and why it's being written. 

Signposting can be accomplished in a sentence or two at the beginning of a paragraph or in whole paragraphs that serve as transitions between one part of the argument and the next. The following example comes from an essay examining how a painting by Monet, The Gare Saint-Lazare: Arrival of a Train, challenges Zola's declarations about Impressionist art. The student writer wonders whether Monet's Impressionism is really as devoted to avoiding "ideas" in favor of direct sense impressions as Zola's claims would seem to suggest. This is the start of the essay's third section:

It is evident in this painting that Monet found his Gare Saint-Lazare motif fascinating at the most fundamental level of the play of light as well as the loftiest level of social relevance. Arrival of a Train explores both extremes of expression. At the fundamental extreme, Monet satisfies the Impressionist objective of capturing the full-spectrum effects of light on a scene.

 The writer signposts this section in the first sentence, reminding readers of the stakes of the essay itself with the simultaneous references to sense impression ("play of light") and intellectual content ("social relevance"). The second sentence follows up on this idea, while the third serves as a topic sentence for the paragraph. The paragraph after that starts off with a topic sentence about the "cultural message" of the painting, something that the signposting sentence predicts by not only reminding readers of the essay's stakes but also, and quite clearly, indicating what the section itself will contain. 

Copyright 2000, Elizabeth Abrams, for the Writing Center at Harvard University

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