Invertion

Inversion of the verb after certain adverbs
In this article, we're going to have a look at inversion that sometimes takes place with certain adverbs and adverbphrases, mostly with a negative or restrictive sense. Such adverbs (adverb phrases) can be placed first in a sentence or clause for emphasis. They are thenfollowed by the interrogative (i.e. inverted) form of the verb.
The most important of these adverbs include:
* hardly ever
* never
* scarcely ever
*only by
* in no circumstances
* only in this way on no account
* hardly . . . when
* only then/when no sooner . . . than
* scarcely . . .when
* not only
* seldom
* nowhere
* not till
* so
* neither/nor
Examples of how to use them:
* I had never before been to lie.=
Never before had I been asked to lie.
* a) / haven't got any money.
b) Neither/Nor have I.
* They not only rob you, they smash everything too. =Not only do they rob you, they smash everything too.
* She became so depressed that. . . =
So depressed did she become that. . .
* This switch must not betouched on any account. =
On no account must this switch be touched.
* Mike didn't realize how difficult how situation was till he received the letter. =Not till he received the letter did he realize how difficult his situation was.
Also note that a second negative verb in a sentence can sometimes be expressed by(nor) with inversion:
* She had no friends and didn't know anyone who could help her. =
She had no friends, nor did she know anyone who could help her.