1. 解释说明
以插入语的形式解释前面提到的概念，它可以给原句提供补充信息，让句子的变得更丰富的同时也变得更容易去理解。
Nevertheless, Pound’s translation conveys the important meaning of the Anglo-Saxon poem and does something that a literal translation fails to do— renders it into poetic English, finding new equivalents for old emotions.(此处在解释Pound’s translation相对于直译的过人之处。)
2.列举事物
给之间出现的概念进行举例，使篇章更加形象生动。
A small glimpse at even some of its subject’s features— her eyes, or perhaps just her hands— brings instant recognition even to those who have no taste or passion for painting. (这里给我们例举some of its subject’s features, 使篇章更具画面感。)
3. 突然转折
由于SAT篇章中表转折意思的破折号常和but等转折词一起连用，所以同学经常会忽视破折号的作用，这里在转折词之前使用破折号会起到强调转折的作用。
In size and mass, Venus is almost the equal of Earth, and its gravitational field is only slightly weaker than ours, so that logically it might be expected to have the same kind of atmosphere— but this is emphatically not so.(这里重点强调之前观点的错误。)
了解阅读中破折号的用法不仅会帮助我们加强对于文章的理解力，而且对于解题会有很大的帮助。
1)功能题，由于破折号具有解释说明的作用，题目可能会直接对破折号的用法进行提问，如
In most people’s minds, yawning— that slow, exaggerated mouth opening with the long, deep inhalation of air, followed by a briefer exhalation— is the most obvious sign of sleepiness.
The comment between the dashes primarily serves to:
clarify a claim
define a term
note a qualification
offer a humorous aside
voice a personal insight
这道题很明显是在考察破折号的作用，这里破折号之间的成份在解释句子主语yawning的意思，所以比较之下选择B项。
2)细节题，有时候句子中的信息不够完全，可以依靠破折号前后的平行关系进行定位或者推测。
This way of looking at cities has recently received the ultimate academic accolade— the creation of a subfield of science, called “urban ecology, ” devoted to understanding it.
The author’s attitude toward the “subfield” is best characterised as one of
approval
curiosity
uncertainty
surprise
dismay
此题询问作者对于subfield的语气，有的同学觉得可以根据creation进行推测，但是此处如果只凭借creation是完全没有说服力的。这时我们可以把目光放到破折号之前，破折号之后的内容意在解释前面的accolade, 所以此处我们应该选择较为正面的选项，即A项。
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