Per esprimere conclusioni su eventi passati, si può usare la forma must have + past participle :
The sidewalks are wet. It must have rained overnight. Our electricity bill is enormous. We must have used a lot of electricity.
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:
Your English has improved. You must have been taking lessons.
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:
Andrea can’t have got my email or he’d have replied by now.
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.