must be
They must be Americans.
Tus textos deben de ser magníficos, quiero leerlos.
Your writing must be magnificent, I want to read it.
— La vida de Adèle (2013)
They must be Americans.
Uds. deben de ser los padres de esta jovencita.
You must be the parents of this young lady.
It must be the hormones.
No deben de ser más de las tres.
It shouldn't be later than three o'clock.