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.
舉個(gè)簡(jiǎn)單的例子，我們用肯定式的"can"的時(shí)候，會(huì)很簡(jiǎn)單的滑過(guò)這個(gè)詞，"can"幾乎不發(fā)音。

They can come on Friday . (stressed words underlined)
他們周五能來(lái)。（有下劃線的單詞應(yīng)該重讀。）

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".
而當(dāng)我們用否定的"can't"的時(shí)候，我們會(huì)加強(qiáng)否定語(yǔ)氣。于是重讀的單詞就變成了：

They can't come on Friday .
他們周五不能來(lái)。

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"說(shuō)起來(lái)就比"They can come on Friday"要長(zhǎng)了。因?yàn)?can't"和"come"都重讀了。

關(guān)于can和can't重讀的問(wèn)題，這里有個(gè)很好的例子。下面一段音頻出自美劇《靈指神探》（Pushing Daises）第一季的第二集（點(diǎn)擊查看學(xué)習(xí)筆記）。

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.（請(qǐng)大家注意聽(tīng)can't的發(fā)音）
（Chuck故意跟了進(jìn)來(lái)）
Ned：Did I say "can"?Because I swallow my consonants sometimes."N'T.N'T." Can't come in.

Ned：我們進(jìn)太平間的時(shí)候你最好呆在車(chē)?yán)?，可能有人?huì)認(rèn)出你的，你真的不能跟我們一起進(jìn)來(lái)。
Ned：我剛說(shuō)的是“can”嗎？因?yàn)槲矣袝r(shí)候說(shuō)話會(huì)吞音。"N'T.N'T." Can't come in.

這里Ned說(shuō)的話如果只靠聽(tīng)力的話可能我們是聽(tīng)不出他說(shuō)的是can't，但是美國(guó)人說(shuō)話如果不是否定式的話can是不會(huì)重讀的，所以我們聽(tīng)這里可以從重讀來(lái)判斷Ned說(shuō)的其實(shí)是否定的can't。這就是重音在溝通中的重要性。

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