50.1.4 Deber (de)+estar+ gerund
This construction expresses supposition with regard to an action in progress. 20.1 (p. 96)
Tu padre debe (de) estar descansando.
Your father must be having a rest.
Deben (de) estar durmiendo.
They must be sleeping.21.2 (p. 100); 48.1.9 (p. 287)
50.1.5 The future, future perfect and conditional in the expression of probability
Supposition or probability with regard to something in the present or the past, can be
expressed with verb forms other than those most frequently associated with the present
and past. Note these examples:
Estará en casa.
He/she must be at home now.
Habrán salido.
They must have gone out.Sería medianoche.
It must have been midnight.17.5.2 (p. 76); 17.6 (p. 77); 17.7 (p. 77); 48.1.10 (p. 287); 48.1.11 (p. 287)
EXPRESSING SUPPOSITION 50.1