主题句是明确地说出段落主要概念或主题的一个句子。 主题句的位置通常是在段落的第一句或开头的几句。 一个清楚、内容充实的主题句有3 个主要的功用:
①告诉读者段落的主题为何。
②让读者能看出段落中所要传达的资讯内容和读者已知的事情有何关联。
③可以为段落中所要传达的资讯设立背景脉络,而段落中的其他句子便由此展开并支持主题句所陈述的重点。
一个好的主题句会让读者大略知道接下来的内容,所以主题句能让读者对段落中所提出的内容先有个准备。 主题句同时也能让读者在浏览全文时立即了解段落的重点,如此一来便能很快地决定是要放慢速度仔细阅读,还是跳过此段直接读下一段落。
科技论文的每个段落通常都应该包含主题句。 一般来说,主题句在段落中的可能位置有3 种:
①主题句可能是一个段落的第一个句子。 科技论文的导言或讨论章节中的段落经常会以主题句开始。
②主题句前面可能会有承接上文的句子或子句。 有时主题句前面会有承接上文的句子,用来连接本段和上一段的内容。 这时,主题句虽然还是出现在段落的前几句,但却不是第一个句子或子句。
③主题句也可能是段落中的最后一个句子。
主题句不仅要告诉读者这个段落将谈些什么,还要能提供一些具体的信息。 比如,假设在开头写这么一个句子:
The structure of the system will be described now．
这并不是一个适当的主题句,因为它所提供的信息太少。 一个较为合适的主题句应该是这样的:
The system consists of two main parts．(www.xing528.com)
请为下列段落选出最适当的主题句,假设所选的句子为段落的第一句。
1．The longer and deeper they stay below, the more of the gas dissolves in their blood．Eventually too much of it is driven into solution by the sea’s greater-than-atmospheric pressure． Subsequently,if a diver returns to the surface without letting his body adjust to the lower pressure, the gas starts bubbling out of the bold and blocking circulation． This causes the bends,the effects of which can be permanently crippling or lethal． In essence, the deeper the dive from the surface, the shorter the stay on the bottom should be．
主题句:
a．Divers must pay careful attention to the effects of nitrogen gas．
b．What ultimately limits divers’ time under water is nitrogen gas．
c． One danger of deep-sea diving is the bends, a phenomenon caused by nitrogen gas dissolved in divers’ blood．
2．They must be longer, relative to the body, than the legs of slower animals, yet they must oscillate very fast． They must also be relatively light, yet strong enough to deliver enormous thrust and to sustain tremendous loads． All of this must be done with sufficient economy of effort to provide endurance． It is evident that nature places greater demands on the mechanical design of horse legs than on the legs of most other animals, even including those of the faster but smaller cheetah．
主题句:
a．The legs of a horse have a unique structure, unmatched in the animal world．
b．The horse’s great running ability coupled with its large size places unusual demands on its legs．
c．In comparison with the legs of a cheetah, a horse’s legs are longer, lighter, and stronger．
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