Certain adverbs of frequency are what explains the structure.
The Cambridge Grammar says it like this:
Some adverbs (e.g. hardly, little, never, only, scarcely and seldom) have a negative meaning. When we use these at the
beginning of the clause, we invert the subject and verb:
Hardly had we left the hotel when it started to pour with rain.
Not: Hardly we had left the hotel
Little did we know that we would never meet again.
Only in spring do we see these lovely little flowers.
We also invert the subject and verb after not + a prepositional phrase
or not + a clause in front position:
Not for a moment did I think I would be offered the job, so I was
amazed when I got it.
Not till I got home did I realise my wallet was missing.
The normal declarative form of the sentence is:
- "The discussion of constitutionalism in public law textbook, if it is discussed at all, never exceeds one page."
If one of those adverbs begins a clause, then, the auxiliary is used with the adverb in front of it followed by the main verb:
- "Never does the discussion of constitutionalism in public law textbook, if it is discussed at all, exceed one page."
In the simple present and simple past of transitive verbs, the auxiliary is put in. In other tenses, the auxiliary is already present.
- Little did he know that we were hiding in the room. [little is a special case ; in declarative form, it has to be said in a different way. "He had no idea we were hiding in the room".]
- Rarely did we go riding on Sunday. FOR: We rarely went riding on Sunday.
Here are some more examples:
From The Economist:
- Seldom has this looked so difficult. FOR: This has seldom looked more difficult. [Auxiliary already present]
From The New York Times:
Seldom has the relationship been more strained. FOR: The relationship has seldom been more strained. [not a transitive, but seldom precedes the verb]
Rarely did they disagree. FOR: They rarely disagreed. [Auxiliary used]
ludwig.guru is a site where you can search newspaper texts.
The structure is usually found in formal writing. Though in Shakespeare's time it was spoken:
A rarer spirit never
Did steer humanity: but you, gods, will give us
Some faults to make us men.
Diomedes, Antony and Cleopatra
Never so much as in a thought unborn
Did I offend your Highness.
Rosalind, As You Like It