INVERSION

SUBJECT-VERB INVERSION 
"Not only is he the principle, he is also my uncle."
This is an example of an inversion in English. Inversion means to put something in an opposite position, and in this post I will be talking its rules when making an English sentence.

Inversion - www.tx.english-ch.com

Let me explain further:

Normally, the structure of a simple sentence would be:
Subject + Verb + Object.
E.g. "She will put the pen down"

But when we use inversion, the structure will look like this, since now the subject and verb has switched places:
Verb + Subject + Object.

The most common form of inversion is question, even though we often don't think about it.
E.g. "She will put the pen down." -> "Will she put the pen down?"

Other than questions, another time when we can use inversion is when there is a negative adverb or adverb phrase (an adverb phrase is two or more words acting as an adverb):
Subject + Verb + Negative Adverb/Adverb Phrase + Noun.
E.g. "I hardly ever eat eggplant."

Now, inversion is when you put everything in the opposite order: adverb/adverb phrase -> verb -> subject -> noun.
Have a look at the inverted version of the example above:
"Hardly do I ever eat eggplant."

Study these examples to understand better:
E.g. "I never work out." -> "Never do I work out in the morning."
E.g. "She seldom says hello to me." -> "Seldom does she say hello to me."
E.g. "He is not only the president but also my best friend." -> "Not only is he the president but he is also my best friend."
E.g. "You should not touch the red button under no circumstances." -> "Under no circumstances should you touch the red button."

Here are a list of some adverb phrases often used in inversion sentences :

Only then

Not only ... but

No sooner

In no way

Only in this way

Only later

On no account

Under no circumstances

Inversion can be used in sentences with "so" and "such":
E.g. "The picture is so beautiful!" -> "So beautiful is the picture!"

E.g. " He is such a good boy." -> "Such good a boy is he."

Last but not least, is inversion in "if" sentences with "had", "should" and "were":
1st conditional:
E.g. "If you need me, I will be in my classroom." -> "Should you need me, I will be in my classroom."
2nd conditional:
E.g. "If I knew the way, we wouldn't be lost now." -> "Had I known the way, we wouldn't be lost now."
3rd conditional:
E.g. "If I had talked to him, he wouldn't have caused a disaster." -> "Had I talked to him, he wouldn't have caused a disaster."