Los nietos ________ aquí porque el salón está sucio. The grandchildren must have been here because the living room is dirty.(HINT: Conjugate "estar" in El Futuro Perfecto.)A suggestion, link this question with the lesson “Using the spanish future perfect to talk about the past”, rather than “Conjugate regular verbs in the perfect future.” Reason: “Must have been” refers to the past. The examples in “Conj. reg verbs in fut. perf.” all refer to the future. Thanks, Shirley.