Per esprimere conclusioni su eventi passati, si può usare la forma must have + past participle :
Andrea’s late. He must have missed the bus again.
Allo stesso modo, si può usare la forma must have been + gerund per concludere su una azione in progresso del passato:
They look really tired. They must have been working hard.
Per esprimere conclusioni negative, suggerimenti che qualcosa è impossibile, si usa can’t have + past participle o can’t have been + gerund:
You can’t have listening or you’d have understood what i was talking about. She can’t have been sick on Friday. I saw her at the party.