一篇文章，或是陳述一個事實，或是解釋一個問題，或是論證一個道理，這就是作者（或文章）的意圖。首先我們來看一看問題的形式：
IV.PURPOSE
1. What is the purpose of the passage?
2. What is the main purpose of the passage?
3. What is the author's main purpose in the passage?
4. The point the writer is making in the passage is that _____.
5. The purpose of this passage is to _______.
6. The author probably intended this selection to ______.

在解答這類問題時，重點在于分析文章的思路，分析作者是如何加工素材的，加工素材的步驟和走向又如何。下面我們來看一篇文章.
IVA.
Convective flow should be familiar to anyone who has noted the boiling of a heated liquid. The most elementary type of convection can be explained by the fact that heat rises. In the simplest cases, convective flow begins
when a fluid is heated from below. As the bottom layer of the fluid is heated, it expands and thus becomes less dense than the layers above. The warmer and lighter bottom layer then tends to rise and the cooler layer
tends to sink in a continuous cycle. The same mechanism of convective flow s responsible for the great ocean currents and for the global circulation of the atmosphere. In an ocean, the water is warmed by the sun to a depth of perhaps thirty meters, and evaporation near the water's surface is responsible for the cooling effect.

The main purpose of this passage is to _______.
A. explain the basic principle of convection
B. describe regular changes in the Earth's atmosphere
C. explain the boiling temperatures of liquids
D. state the principles of ocean current

這篇文章講述的是對流convective flow。仔細分析一下作者的思路和選材，第一句話是引導句，從第二句話開始，就以液體為例，講述對流的機理mechanism of convective flow,所以作者的目的就是解釋對流的原理，答案應該選A.可見，在解答此類問題時，和前面一樣，還是要設法抓住主題句。在看一個例子：
IVB.
Although commonly thought to be one of man's feared deepsea enemy, the octopus(章魚) is generally a harmless animal that rarely bothers man. In fact, most of this powerful, eightarmed creatures are afraid of people.
There have been some cases in which octopuses have attacked divers. But even these scattered attacks have not been very serious. In the mid nineteenth century, Hugo is said to have started the idea that the octopus
is a vicious monster of the deep. In this "Toilers of the Sea" he described how this devil fish eats a human being. The tale became so popular that other novelists, and later the movies used episodes which describe man struggling in the arms of this frightening monster. And thus, the misconception of the octopus as a vicious, merciless killer was spread.
The purpose of this passage is to ______.
A. describes the stories of the octopus
B. explain the causes of the misconception of the octopus
C. prove the harmlessness of the octopus
D. correct the misconception of the octopus

這篇文章的題目出的很好，有較強的干擾性，正確答案應該是D, 有沒有選對？下面我們來分析一下。我想這篇文章可以分成三個部分：
1. 第一句話
2. from "In fact,..." to "...very serious".
3. from "In the mid..." to end.
第一部分顯然是主題句, 第二部分證明章魚是無害的, 第三部分解釋對章魚偏見的來由.選項中B 和C 只是概括了文章的部分內容, 只有D才是最符合文章主題句的.最后一篇文章供你們練習,有點難度,但只要抓住主題句,還是能夠選對的.

IVC.
Astronomers say Arizona's observatories are being jeopardized by the state's burgeoning population because artificial lights glow steadily brighter and make the stars harder to see. Research has already been
severely impaired a the Lowell observatory in Flagstaff, Arizona, by the lights of the encircling city of 39,000. Each year, the observatory's telescopes find it more difficult to penetrate the artificial lights. As a result, astronomers are working with local officials to dim the glow with restrictions on searchlights and requirements for light shields. Mercury-based streetlights, the most common type in cities in the United
States, are especially disturbing to stargazers (天文學家). An alternative would be sodium lights, which emit no ultraviolet (紫外線的) waveband free the ultraviolet band that is essential to spectral astronomy.

What is the purpose of the passage?
A. To explain recent research
B. To advertise for better telescopes
C. To criticize current lighting system
D. To report on an existing situation
答案是D, 主題句是第一句話。