If reference is to the past, deber (de) must be followed by haber and a past participle.Deben (de) haber regresado ya.
They must have returned already.Debe (de) haber llamado.
He/she must have phoned.Debe (de) haberse molestado.
He/she must have got annoyed.^50 (p. 296)
48.1.9 Deber (de)+estar+ gerund
This construction with estar+ gerund (see 20.1) is used when the notion of probability
or supposition refers to an action in progress.Debe (de) estar trabajando. He/she must be working.
Deben (de) estar peleando. They must be having a row.If reference is to the past, deber (de) must be followed by haber+ past participle of
estar+ a gerund.Deben (de) haber estado esperando.
They must have been waiting.Debe (de) haber estado nadando.
He/she must have been swimming.^50 (p. 296); 21.2 (p. 100); 20.1 (p. 96)
48.1.10 Future
Probability with regard to something in the present is sometimes expressed with the
future. Compare these sentences in which the second of each pair expresses probability
while the first indicates certainty:Está con alguien.
He/she is with someone.Estará con alguien.
He/she must be with someone.Ya van camino del/al trabajo.
They are on their way to work already.
Ya irán camino del/al trabajo.
They must be on their way to work already.17.5.2 (p. 76);^50 (p. 296)
48.1.11 Future perfect
Probability with regard to something in the past, but which bears some relationship
with the present is expressed with the future perfect. Consider these examples:Saying whether something is considered possible or impossible 48.1