We use inversion to emphasise the meaning of a sentence.
It’s used after expressions such as ‘only then‘, ‘after‘, ‘rarely‘, ‘little‘ and ‘never‘.
We use the present and past perfect.

Examples :

– Never have I seen a tiger. Never had I seen a tiger.
– Rarely have I seen so many tigers. Never had I seen so many tigers.
– Only then had I realised it was too late.