What this form does
English guesses with an extra word: must, probably, I bet. Spanish shifts the tense one step into the future instead: the futuro guesses about the present, the condicional, as the future seen from the past, about the past. You know the form as would; here it means must have been.
Now and then
Estará en casa a estas horas.
He must be home at this hour. (now)
Estaría en casa cuando lo llamé.
He must have been home when I called him. (then)
What you use it for
Estimating
A time, an age, a quantity or a state that you did not know for sure back then.
Serían las diez cuando empezó la reunión.
It must have been ten o'clock when the meeting started.
Tendría unos treinta años cuando se casó.
She must have been about thirty when she got married.
Ayer no vinieron: estarían cansados.
They didn't come yesterday: they were probably tired.
Wondering
In questions the condicional carries the flavour of English I wonder… or why on earth…: puzzlement about something that already happened.
¿Por qué lo haría?
I wonder why he did that.
¿Qué querría decir con eso?
What was he trying to say by that, I wonder?
Ayer no lo vi. ¿Dónde estaría?
I didn't see him yesterday. Where could he have been?
