Se forma el condicional II con el simple past de los verbos modales de will o shall o sea would o should.

Condicional II
would / should + present perfect condición
I would / should have gone if I had known about it.
Yo habría ido si lo hubiera sabido
We would / should have left if nobody else had come to the party.
Nosotros habríamos ido si nadie más habría andado a la fiesta.
He would have cooked if he had been able to
Él habría cocinado si le hubiera sido posible.
You would have slept if only you had had time.
Tú habrías dormido si hubieras tenido tiempo.
They would have written if they had had paper.
Habrían escrito si hubieran tenido papel.