La inversión es un recurso de nivel alto para dar énfasis y formalidad: cuando una frase empieza por una expresión negativa o restrictiva (never, rarely, hardly, not only, under no circumstances), el orden se invierte como en una pregunta — el auxiliar va antes del sujeto. I had never seen that se vuelve *Never had I seen that*. Suena literario y sube mucho la nota en writing, pero solo funciona si el detonante negativo va al principio.
Detonantes frecuentes
Never / Rarely / Seldom + auxiliar + sujeto: *Never have I felt so proud.*
Hardly... when / No sooner... than: *Hardly had I arrived when it rained.*
Not only... but also: *Not only does he sing, but he also dances.*
Under no circumstances / On no account / In no way + auxiliar + sujeto: *Under no circumstances will I agree.*
Only... (la inversión va en la segunda parte)
Only after / only by / only when / only if: Only after finishing can you go out.
Only then / Not until: Only then did they realise. / Not until she arrived did we start.
Condicionales sin "if"
EN1st: *Should you need help, call me.*
ES1º: *Should you need help, call me.*
EN2nd: *Were I rich, I'd travel.*
ES2º: *Were I rich, I'd travel.*
EN3rd: *Had I known, I would have come.*
ES3º: *Had I known, I would have come.*
El error típico del hispanohablante
Si pones el negativo delante, TIENES que invertir: *Never have I*, no "Never I have".