Let's look at a simple example: the modal verb "can". When we use the positive form of "can" we quickly glide over the can and it is hardly pronounced.
举个简单的例子，我们用肯定式的"can"的时候，会很简单的滑过这个词，"can"几乎不发音。

They can come on Friday . (stressed words underlined)
他们周五能来。（有下划线的单词应该重读。）

On the other hand, when we use the negative form "can't" we tend to stress the fact that it is the negative form by also stressing "can't".
而当我们用否定的"can't"的时候，我们会加强否定语气。于是重读的单词就变成了：

They can't come on Friday .
他们周五不能来。

As you can see from the above example the sentence, "They can't come on Friday" is longer than "They can come on Friday" because both the modal "can't" and the verb "come" are stressed.
所以大家就知道，上面的例句中，"They can't come on Friday"说起来就比"They can come on Friday"要长了。因为"can't"和"come"都重读了。

关于can和can't重读的问题，这里有个很好的例子。下面一段音频出自美剧《灵指神探》（Pushing Daises）第一季的第二集（点击查看学习笔记）。

Ned：So it's probably better if you stay in the car for these morgue visits. Someone might recognize you.You really can't come in.（请大家注意听can't的发音）
（Chuck故意跟了进来）
Ned：Did I say "can"?Because I swallow my consonants sometimes."N'T.N'T." Can't come in.

Ned：我们进太平间的时候你最好呆在车里，可能有人会认出你的，你真的不能跟我们一起进来。
Ned：我刚说的是“can”吗？因为我有时候说话会吞音。"N'T.N'T." Can't come in.

这里Ned说的话如果只靠听力的话可能我们是听不出他说的是can't，但是美国人说话如果不是否定式的话can是不会重读的，所以我们听这里可以从重读来判断Ned说的其实是否定的can't。这就是重音在沟通中的重要性。

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