We use the reversal of word order for emphasis the construction.
Let's see those two expamples:
Carl has never been so aggressive.
Never has Carl been so aggressive (emphasizing never).
In example 2 the adverb never was used at the beginning of the sentence and the order of subject and verb was reversed. Instead of Carl has never, the construction Never has Carl was used. This kind of construction is used for emphasis.
The intonation cannot be expressed in writting. Thus, for the purpose of emphasis, we can place certain adverbs, adverbial expressions, and negative conjunctions at the beginning of a sentence. As a result, a reversal of word order occurs.
Let's check some examples common cases in which the order of subject and verb is reversed.
A - After adverbial expressions of negation with no, not, never, neither.
In no/neither case would I accept your offer.
On no account should you accept her help.
Not until he gets sick will he stop drinking.
Never has she seen such a stupid person.
B- After adverbial expressions that have a negative effect:
Little did she realize how she had hurt my feelings.
Seldom/Rarely does Tom mean what he saysm .Little did she realize how she had hurt my feelings.
C- After adverbial expressions with only:
Only after a three-hour delay did their flight leave.
Only later did I recognize him.
D- After negative conjuctions such as not only, no sooner...than:
Not only is Carl being sarcastic, he`s being ironic as well.
No sooner had he begun his trip than he became sick.
The word order is also reversed:
*in questions (Can I talk to you for a moment?)
*after the conjuction so (I work until five and so does my wife)
*in exclamations (Isn`t it cold!)
*after adverbial expressions of place (There stood the tallest woman I`ve ever seen)
See you!