No sooner had he arrived than he fell ill

[Tamara]

Hi

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)?
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It’s impossible to learn swimming without entering the water…

[Mister Micawber]

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#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.
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Native English teacher at Mister Micawber's

[Tamara]

[Mister Micawber]

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Yes. 'Negatives or negative-like expressions' I think is the phrase often used.
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Native English teacher at Mister Micawber's