tate region of the midwest, including Ohio. McCarthy (1995) documented 50% hickory mortality, associated with insects, in a ten year study of forest dynamics in two hardwood areas in southeastern Ohio forest. The lack of decline reports for oak-hickory vegetation in Ohio may simply be because the forest is essentially healthy. Alternatively, decline symptoms may be present but not reported due to a shortage of studies explicitly designed to evaluate decline in certain forest types.[5] The purpose of this study was to conduct a preliminary forest health study of a representative oak-hickory forest of southeastern Ohio. A decline index was employed to categorize present symptoms and predict future dynamics while mortality was used as an indicator of past decline and its resultant effects on present forest dynamics. 第二层(第4段)：指出研究的空白。关于俄亥俄州橡树与山核桃树减少与死亡的研究报告却很少见。第三层(第5段)：说明本研究项目的目的。为了填补上述空白，本文对俄亥俄州东南部地区的橡树与山核桃树的健康状况进行初步研究。
可见，引言一般分为三到四个层次。每个层次都有各自的任务与目的。而在语言上也有各自的特点，掌握这些特点会使写作过程化难为易。下面将对引言各层次的写作特点和技巧分别加以介绍。
8.2 如何写引言的开头
引言开头(即第一层)最主要目的是告诉读者论文所涉及的研究领域及其意义是什么，研究要解决什么问题，目前状况或水平如何。也就是说，开头要回答如下问题：
l What is the subject of the research?
l What is the importance of this subject?
l How is the research going at present?
l In what way is it important, interesting, and worth studying?
l What problem does the research solve?
下面列举几段例子，以说明引言开头如何开门见山点明研究主题，回答以上问题。
例3
Purple loosestrife (紫千曲菜) is a wetland plant which invaded North America in the early 1800s. It is well established in the United States and Canada with serious infestations in the Great Lakes and St. Lawrence regions.
研究主题是purple loosestrife。这种植物之所以值得研究，是因为它严重侵扰了五大湖区及圣劳伦斯地区。
例4
Forecast of the tracks of hurricanes (飓风)have improved steadily over the past three decades, owing to a combination of better observations and much improved numerical models. These improvements, along with advances in warning systems and preparedness for emergencies, have brought about a significant decline in loss of life.
研究主题是 forecast of the tracks of hurricanes(对飓风轨道的预测)。这一领域取得了进步，意义是大大减少人员伤亡损失。
例5
Environmental indicators(环境指数)attempt to accurately describe environmental conditions and make available scientifically valid information on environmental trends．
研究主题是environmental indicators。环境指数的意义是能够精确地描述环境状况，并能够提供关于环境倾向的科学、有效的信息。
例6
Acoustic problem occur extensively in long spaces. For example, in road or rail tunnels noise pollution is a serious problem, and in underground stations poor speech intelligibility of public address systems can cause misunderstanding of vital instructions during an emergency.
研究主题是acoustic problem of longs paces(长空间的声学问题)，例如，公路与铁路隧道上的噪声污染就是一个严重的问题。地铁站的广播系统语言清晰度差，造成在紧急状态下信息误解，因此，这个问题值得研究解决。
通过分析例3到例6，还可以发现如下特点。
首先，关键词往往出现在第一句话，回答What is the research area? 这个问题，迅速将主题告诉读者，避免转弯抹角，影响信息传递。然后简单介绍该研究领域的意义。
第二，引言开头句子的谓语动词或者是一般现在时态，或者是现在完成时态。这是因为这些动词所描述的是某研究领域的现状，而不是过去的情况。
引言的开头常用句型还有：
句型1：研究主题+谓语动词be… ．
例7
a．Fuel cell(燃料电池)is a technology for the clean and efficient conversion from chemical energy in fossil fuels to electricity．
b. Spot welding (点焊) is the most widely used joining method in the automobile manufacturing industry.
c. Accurate prediction of springback (回弹预测) for sheet metal forming process is vital to the design of tools used in automotive sheet stamping operations.
句型2: 研究主题

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