Question

as soon as he sat on the chair the chair broke (begin hardly)​

Answer 1

Hardly had he sitten on the chair when the chair broke.

Answer 2

Q) As soon as he sat on the chair the chair broke. (begin hardly)

--> Hardly had he sat on the chair when it broke.

Using hardly the format is

subject + v² (2nd firm of verb ) + object

add hardly

Hardly + had + subject + v² + object + when + clause