To talk about something that would, could, or should have happened—but didn’t—you need the conditional perfect, also known as the past conditional.
The conditional perfect is commonly used in si clauses: the conditional perfect explains what would or would not have happened, an action that was dependent upon something else happening or not happening (which is indicated by the pluperfect subjunctive).
|Si hubiera sabido la hora de tu vuelo, te habría encontrado al aeropuerto.||If I had known your flight time, I would have found you at the airport.|
|¿Habrías comprado el libro si te hubiera dicho?||Would you have bought the book if I had told you?|
As you can see, both of these theoretical actions are in the past, and neither one occurred. I didn’t know your flight time, so didn’t find you at the airport, and since I didn’t tell you, you didn’t buy the book.
The conditional perfect can also be used without a dependent action, such as to express regret or to criticize.
|Habría comprado el café pero no tenía tiempo.||I would have bought coffee but I didn’t have time.|
|¿Qué habrías hecho, en mi lugar?||What would you have done, in my position / shoes?|
In all of the above, the conditional perfect is used the same way in French and English.
But there’s one construction in which the condicional perfecto is used only in Spanish: to report a probability or guess.
|Lucas habría comido antes de salir.||Lucas had probably already eaten.|
|Habrían sido las dos cuando llegamos.||It must have been 2 o’clock when we arrived.|