信息结构是把语言组织成信息单位（information unit）的结构。每一个信息单位是由已知信息（given information）和新信息（new information）组织而成。语篇中的已知信息提供交际所需的背景，而新信息则在已知信息的基础上创造出信息差，信息差决定了语篇信息的交际动力。如果数个小句主位包含的全部都是新信息，那就构建不成语篇。如果全部都是旧信息，语篇便得不到发展。一般来说，主位表达已知信息，是信息的起始点。而述位则表达新信息，是信息的中心，这也是英语句子“信息焦点尾重原则”的反映。但并不总是如此，有时为了达到特殊的表达效果，也会用主位来表达新信息。在书面语篇中，由于无法用语调来表明重要信息，所以一般要依靠语序来区分主、次信息。例如：
（1）Milgram’s experiment（T1）solves an important question in sociobiology（R1）.
（2）An important question in sociobiology(T2) is solved by Milgram’s experiment(R2).
这两个句子中，主位和述位的不同决定了已知信息和新信息的不同，从而使得第一句的信息中心是an important question in sociobiology，而第二句的信息中心是Milgram’s experiment。
3．衔接系统
三．语篇功能与雅思阅读答题思路
1．主位推进及信息结构与段落主旨题和摘要题的答题策略
单个句子都有自己相对独立的主位结构。但是语篇中的主述位并不是孤立出现的，前后句子的主位和主位、述位和述位、主位和述位之间始终存在某种有机的联系和变化，这种联系和变化就叫推进（progression）。随着各句主位的向前推进，整个语篇逐步展开，直至形成一个能表达某一完整意义的整体（朱永生，2001）。对于语篇的主位推进模式，国内外语言学家提出了很多不同的分类。其中主要有五种模式：
（1）主位同一型（亦称平衡型）：主位（部分）相同，述位不同
T1+R1→T2(=T1)+R2→T3(=T1)+R3
（2）述位同一型（亦称集中型）：主位不同，述位（部分）相同
雅思阅读涉及语篇主旨的主要题型是List of Headings（选配段落标题），考查对文章中各段中心思想的把握。答题可利用最有可能是主题句的首次尾中句分别与标题选项逐一对应，选配最佳答案。而在段落没有主题句的情况下，则可以通过分析语篇的主位推进模式，了解其信息结构和分布动态，把握作者的思路流程或序列，并结合该题型的命题特点，重点关注主、述位信息链条上的逻辑关系切换、信息衔接上的重复（或强调）等环节，从而准确地解读作者在语篇中要表达的主要意思。而对于Summary（摘要题），题干语篇的主位推进模式也会为答案的语意预判提供一定的线索。已有学者对中外学术期刊文章摘要的主位推进模式做过分析统计，结果是派生型居多。而笔者对雅思摘要题题干的分析统计表明，其主位推进模式以延续型、集中型和平衡型为主。
例1．剑桥5/Test3/Reading Passage2/Q15 Paragraph D (List of Headings)
Choose the correct heading for paragraphs B and D-F from the list of headings.
Paragraph D（共4个句子：S1-S4）
(S1)Once north of Cairo,[复项主位T1]‖most of the Nile water is diverted into more than 10,000 kilometers of irrigation canals and only a small proportion reaches the sea directly through the rivers in the delta[述位R1].(S2)The water in the irrigation canals[单项主位T2（=R1）]‖is still or very slow-moving and thus cannot carry sediment, Stanley explains[述位R2].(S3)The sediment[单项主位T3（=R2）]‖sinks to the bottom of the canals and then is added to fields by farmers or pumped with the water into the four large freshwater lagoons that are located near the outer edges of the delta[述位R3].(S4)So very little of it[复项主位T4（=T3）]‖actually reaches the coastline to replace what is being washed away by the Mediterranean currents[述位R4].
第一句的述位（water is diverted into irrigation canals）是第二句的主位（water in the irrigation canals），第二句的述位（cannot carry sediment）是第三句的主位（The sediment），第四句的主位（very little of it）和第三句的主位相同。这种延续型和平衡型主位推进模式构建了该语篇的语意框架，表明了语篇中的信息流程和所要传递的信息焦点，即关于irrigation canals和sediment的相互关系。因此可以判定标题选项“Effects of irrigation on sedimentation”为正确答案。
例2．剑桥4/Test4/Reading Passage3/Q31 Section E (List of Headings)
Choose the correct heading for sections A and C-E from the list of headings.
(S1)Just as the time when it became obvious that health-care resources could not possibly meet the demands being made upon them,[句项主位T1]‖people were demanding that their fundamental right to health-care be satisfied by the state[述位R1].(S2)The second set of more specific changes that have led to the present concern about the distribution of health-care resources[句项主位T2(=T1＊)]‖stems from the dramatic rise in health costs in most OECD countries, accompanied by large-scale demographic and social changes which have meant, to take one example, that elderly people are now major (and relatively very expensive) consumers of health-care resources[述位R2].(S3)Thus in OECD countries as a whole,[复项主位T3]‖health costs increased from 3.8% of GDP in 1960 to 7% of GDP in 1980, and it has been predicted that the proportion of health costs to GDP will continue to increase.(In theUSthe current figure is about 12% of GDP, and inAustraliaabout 7.8% of GDP.)[述位R3(=R2)]
(S4)As a consequence, during the 1980s[复项主位T4]‖a kind of doomsday scenario (analogous to similar doomsday extrapolations about energy needs and fossil fuels or about population increases) was projected by health administrators, economists and politicians[述位R4(=R2＊)].(S5)In this scenario,[单项主位T5（=R4）]‖ever-rising health costs were matched against static or declining resources[述位R5(=R3＊)].
（1）主位推进模式分析：
T1+R1→T2(=T1＊)+R2‖[派生型]→T3+R3(=R2)‖[集中型]→T4+R4(=R2＊)‖[派生型]
（2）信息衔接上的重复（或强调）：S2→S3的主位推进是集中型，述位（R
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