No sooner had he arrived than he fell ill


I am trying to learn the (a bit too ‘spagetti’, as to me) form with Past Perfect + ‘no sooner’.

But the question is about the style. This is the example to illustrate it:

  1. He had no sooner arrived than he fell ill.
  2. No sooner had he arrived than he fell ill.

Why is the inversion had he in the second case?

I was only told that, for example, Had I been there, I would have helped her.
is more formal (style) than If I had been there, …

Is this the case (of just different level of formality)?

#2 is an example of another phenomenon-- inversion after an initial negative element:

No sooner had he arrived than he fell ill.
Never have I seen such a mess.
Seldom do we see fishermen making their wives bait the hooks

It is a matter of placing stress in the sentence; I suppose (supposingly?) we might call it more dramatic.

So (supposingly? :)), negative element you mentioned is defined in terms of its contents (semantics),
not just by ‘pure grammar’ (syntax) structure (= explicit presence of ‘no’, ‘not’,… ) ?

Thank you.

Yes. ‘Negatives or negative-like expressions’ I think is the phrase often used.