很多同学应该都学过一个经典句式：not only…but (also)…这一句式可以用来表示并列及递进关系，在地道英文中很常见，比如新概念3中就有这样的句子：

We live in dread not only of unpleasant insects like spiders or wasps, but of quite harmless ones like moths. 

（我们在日常生活中，不但憎恶如蜘蛛、黄蜂之类令人讨厌的昆虫，而且憎恶并无大害的飞蛾等。）

这里but (also)后面接的内容要比not only后面的内容在意思上更进一步（The second thing mentioned is usually more surprising, informative, or important than the first one.），这一句式虽然看起来简单，但是要用好它并不容易。很多同学在使用的时候经常出现各种小问题，其中最常见的问题是忽略了句式的平行结构。

所谓的平行结构是指not only以及but also后面的结构和词性要保持一致，比如名词对名词，动词对动词等，比如上面新概念3的句子中of unpleasant insects…以及of quite harmless one…结构就是一致的。如果两部分结构不一致就会出现失衡，举个例子：

He is not only intelligent but also has a great sense of humor.

这一句子违反了平行结构的原则，因为划线的两部分词性不一致，可以改为：

He is not only intelligent but also funny.

再看一个复杂一点的例子：

I not only knew where this person was shopping and how much he or she was spending, but the exact time of each transaction.

上面句子中划线的两个部分也不平衡，为了保持平行结构，我们可以写成：

I not only knew where this person was shopping and how much he or she was spending, but knew the exact time of each transaction.

但这样写会出现两个knew，显得累赘，更好的解决方式是把knew提到not only之前，这样动词可以同时作用于not only…but…also…后面两个结构，即把“主语+not only…+动词+but also…”改为“主语+动词+not only…+but also…”

I knew not only where this person was shopping and how much he or she was spending but also the exact time of each transaction.

not only…but also…句式的变体

not only…but also…句式还有其他变体，比如当not only…but (also)…后面都接动词的时候，我们可以用代词来代替but (also)…这部分，举个例子：

Her interest in your work has not only continued but also increased.

这一句式可以改为：

Her interest in your work has not only continued, it has increased.

又比如这一个BBC句子：

…older women are not only less represented in the workforce, they are less likely to be hired than an older man, according to the American Society on Ageing.

这一句子也可以改写为：

…older women are not only less represented in the workforce but also less likely to be hired than an older man, according to the American Society on Ageing.

not only…but also…的倒装句

有时候为了起到强调作用，我们会使用not only…but also…的倒装形式，其基本格式为：not only+助动词/be动词+主语+动词，举个例子：

They not only sent home substantial earnings but also saved money. 

这句话相当于：

They not only sent home substantial earnings, they also saved money.

改为倒装句之后是：

Not only did they send home substantial earnings, (but) they also saved money.

又比如这一个BBC例句：

Not only did they oversee the spread of good government, western education, modern medicine and the rule of law, they also put in place local works, famine relief, and irrigation projects...

还原后是：

They not only oversaw the spread of good government, western education, modern medicine and the rule of law but also put in place local works, famine relief, and irrigation projects...

除了上面介绍的这几点外，还要注意but (also)前面的逗号问题，but (also)前面加不加逗号都可以，不过有时候为了强调会加逗号，比如下面这条《麦克米伦词典》例句：

There are problems not only with the students, but also with the teachers themselves.

《韦氏高阶英语词典》的这条例句则没有加逗号：

The festivals take place not only in the city but also in the surrounding rural areas.

参考资料：

[1]https://www.grammarly.com/blog/parallelism-with-not-only-but-also/

[2]https://dictionary.cambridge.org/grammar/british-grammar/not-only-but-also

[3]https://www.dailywritingtips.com/5-tips-for-fixing-%E2%80%9Cnot-only-but-also%E2%80%9D-errors/

[4]Collins COBUILD English Usage