例4

　　Researchers have established that when people are mentally engaged, biochemical changes occur in the brain that allow it to act more effectively in cognitive(認(rèn)知的)ar-eas such as attention and memory. This is true regardless of age.

　　People will be alert(警覺的)and receptive(愿意接受的)if they are faced with information that gets them to think about things they are interested in. And someone with a history of doing more rather than less will go into old age more cognitively sound than someone who has not had an active mind.

　　Many experts are so convinced of the benefits of challenging the brain that they are putting the theory to work in their own lives. "The idea is not necessary to learn to memorize enormous amounts of information,"says James Fozard, associate director of the National Institute on Aging. "Most of us don't need that kind of skill. Such specif- ic training is of less interest than being able to maintain mental alertness. " Fozard and others say they challenge their brains with different mental skills, both because they enjoy them and because they are sure that their range of activities will help the way their brains work.

　　Gene Cohen, acting director of the same institute, suggests that people in their old age should engage in mental and physical activities individually as well as in groups. Cohen says that we are frequently advised to keep physically active as we age, but older people need to keep mentally active as well. Those who do are more likely to maintain their intellectual abilities and to be generally happier and better adjusted, "The point is, you need to do both," Cohen says. "Intellectual activity actually influ-ences brain-cell health and size. "

　　本文以引經(jīng)據(jù)典的方法強(qiáng)調(diào)了頭腦健康，尤其是老年人頭腦健康的重要性。這個(gè)中心內(nèi)容在第一段第一句就已指出：人在用腦時(shí)，大腦會處于興奮狀態(tài)，這時(shí)大腦在諸如注意力和記憶力等屬于認(rèn)知領(lǐng)域的能力方面會得到有效發(fā)揮。

　　What is the passage mainly about?

　　A. How biochemical changes occur in the human brain.

　　B. Why people should keep active not only physically but also mentally.

　　C. How intellectual activities influence brain-cell health.

　　D. Why people should receive special mental training as they age.

　　這道題的正確答案為B.A與C均為原文中的細(xì)節(jié)，而D與原文第三段第4～5行"...Such specific training is of less interest than being able to maintain mental alert-ness"相矛盾，所以都不是正確答案。

　　(2)主題句在第一段末句

　　首段末句也是主題句常出現(xiàn)的地方。假若如此，這個(gè)主題句就不僅是對全文的總結(jié)，也是對第一段的總結(jié)，而且開啟第二段及以后的說明或論述。簡而言之，這種文章結(jié)構(gòu)是：具體說明或論述(一段)一總結(jié)(一段末的主題句)一后文的具體說明或論述。

　　這種形式的主題句之前通常有表示總結(jié)的提示詞，如：in conclusion, to conclude, in summary, to sum up, in short, in brief, in a nutshell;或表示推斷的提示詞：there-fore, thus, as a result, accordingly;或表示轉(zhuǎn)折的提示詞：but, however等。轉(zhuǎn)折之后就出現(xiàn)主題句的情況較多，這種謀篇套路可以用“去舊取新、標(biāo)新立異”八個(gè)字概括。文章中諸如"every parent"、"a popular belief"、"frequently assumed"、"universally accept-ed"等詞句提示作者可能在下文提出一個(gè)與之不同的觀點(diǎn)標(biāo)新立異，假若如此，那么作者“標(biāo)新立異”之處的觀點(diǎn)就是主題大意。對于某一現(xiàn)象，過去已有解釋，作者提出新的解釋，新解釋就是主題大意。這體現(xiàn)了上述的“去舊取新”原則。

　　例5

　　Do you find getting up in the morning so difficult that it's painful? This might be called laziness, but Dr. Kleitman has a new explanation. He has proved that everyone has a daily energy cycle.

　　During the hours when you labour through your work you may say that you're 'hot’. That's true. The time of day when you feel most energetic is when your cycle of body temperature is at its peak. For some people the peak comes during the fore- noon. For others it comes in the afternoon or evening. No one has discovered why this is so, but it leads to such familiar monologues(自言自語)as: 'Get up, John! You'll be late for work again!' The possible explanation to the trouble is that John is at his temperature-and-energy peak in the evening. Much family quarrelling ends when hus- bands and wives realize what these energy cycles mean, and which cycle each member of the family has.

　　You can't change your energy cycle, but you can learn to make your life fit it bet- ter. Habit can help. Dr. Kleitman believes. Maybe you're sleepy in the evening but feel you must stay up late anyway. Counteract(對抗)your cycle to some extent by ha- bitually staying up later than you want to. If your energy is low in the morning but you have an important job to do early in the day, rise before your usual hour. This won't change your cycle, but you'll get up steam and work better at your low point.

　　Get off to a slow start which saves your energy. Get up with a leisurely yawn(呵欠)and stretch. Sit on the edge of the bed a minute before putting your feet on the floor. Avoid the troublesome search for clean elothes by laying them out the night be-fore. Whenever possible, do routine work in the afternoon and save tasks requiring more energy or concentration for your sharper hours.

　　If a person finds getting up early a problem, most probably

　　A. he is a lazy person

　　B. he refuses to follow his own energy cyde

　　C. he is not sure when his energy is how

　　D. he is at his peak in the afternoon or evening.

　　這段文章先提出現(xiàn)象：早晨起床困難，接著以不肯定的口吻提出一種解釋，然后"but"一轉(zhuǎn)，提出Kleitman博士的新解釋：各人的精力在一天之中有個(gè)周期。這個(gè)"but"之后的新解釋就是主題句。

　　(3)文章首末兩段均有主題句

　　主題句到末段才出現(xiàn)的情況較少，常見的情況是文章開門見山提出主題句，中間加以論述，文末復(fù)述首段的主題，這也是寫作方法之一。