El Condicional Simple is used to express conjecture about the past. We use it to express what we think was probably happening/happened "back" in the past.
Have a look at the following examples:
If Person 1 says:
Son las 11 de la mañana, y Antonio todavía está durmiendo.
It is 11 am, and Antonio is still sleeping.
Person 2 might ask:
¿A qué hora llegaría a casa anoche?What time did he get home last night? [I wonder what time he returned home last night?]
Then Person 1 could reply:
Yo creo que llegaría a casa sobre las 3 de la mañana.I think he probably got home at about 3 in the morning.
By using El Condicional Simple here, both speakers are speculating about what happened last night. They are not stating known facts, only what they think probably happened.
To state known facts we use El Pretérito Indefinido and not El Condicional Simple:
Llegó a las 3 de la mañana.
He arrived at 3 in the morning.
Here are more examples using El Condicional Simple for the same purpose:
¿Cuándo saldría el tren?When did the train leave? [I wonder what time the train left?]
Serían las siete cuando terminó por fin la reunión.It was probably seven o’clock when the meeting finally finished.
Tendríamos diez años en esa foto.We were probably ten years old in that photo.
Cuando yo la vi por última vez, tendría unos 20 años.When I last saw her, she was probably about 20 years old.
SUMMARY: In Spanish you use El Condicional Simple to speculate about something that happened in the past, to express probability with regards to a past action.
If the conjecture or expression of probability is about the present, then you use El Futuro Simple. See Using El Futuro Simple (simple future) for prediction, speculation and probability in the present.
See also Using El Condicional to talk about a future in the past.
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