I've already talked about how some phrases function as splitters—how what goes into either side must look the same. One of the phrases I mentioned as a splitter phrase was "Not only ... but also." But there's a funny bit of construction with this phrase, so let's look at the different ways you can use it. Suppose I want to say that "This fruit is both juicy and delicious." But I want to do it with a "not only ... but also" phrase.
According to our splitters rule, we need to construct it so either side can plug in to the beginning of the sentence on its own - meaning, if we take out the splitter and plug in one piece on its own, it still makes a good sentence.
We do that above with the "both"/"and" construction: The fruit is [both] juicy. The fruit is [and] delicious. We can do the same with "not only ... but also":
The fruit is [not only] juicy. The fruit is [but also] delicious. To create our sentence, we read the split text one line at a time: The fruit is not only juicy, but also delicious. Suppose, though, that "is" doesn't apply to both parts of our sentence. Suppose we wanted to say
That doesn't break up well: The fruit is [not only] juicy. The fruit is [but also] tastes delicious. "The fruit is tastes delicious." Ummmm.... How do we solve this? Move the "is" until after the splitter.
"All right, I get that!" you say. "This is just another example of splitters! What's so special about not only ... but also?"
Glad you asked. Turns out there's a third way to make this sentence work, but it's not as intuitive as the others:
Not only is the fruit juicy, but it also tastes delicious. Obviously, "It [also] tastes delicious" is a fine sentence. But "Is the fruit juicy" isn't exactly what we're trying to say. Still, in this case, that's how you write the sentence. A bit topsy-turvy, but all the same parts are on both sides.
It's a pretty simple balancing act (fear my Word art skills), but it's amazing how often writers will give me sentences like: Not only is the fruit juicy, but also delicious. Not only is the fruit juicy, but also tastes delicious. These are wrong! The whole sentence "The fruit is juicy" appears inside the "Not only" construction, so you need another whole sentence to match it in the "but also." Clear as mud? Just remember this as a rule of thumb: every piece that comes after the "not only" also has to come after (or within) the "but also." The two parts of the sentence should "weigh" the same.
Go forth and write well-balanced sentences!