El third conditional imagina un pasado distinto del que ocurrió. Ya no se puede cambiar: solo imaginar.

if + had + participio, would have + participio

• If I had known, I would have come earlier. — Si lo hubiera sabido, habría venido antes.
• If we had taken the train, we would have arrived on time. — Si hubiéramos cogido el tren, habríamos llegado a tiempo.

Had + participio es el past perfect (hubiera o había + participio). En los verbos regulares, el participio acaba en -ed, como el pasado: missed, invited, stayed. Los irregulares más útiles:

Infinitivo	Pasado	Participio
go	went	gone
know	knew	known
meet	met	met
take	took	taken
see	saw	seen
come	came	come
be	was / were	been
get	got	got (en EE. UU., gotten)