Certainty about the present
•We use must + infinitive (without to) if we are sure something is true
👉 He is a biologist, he must know a lot about animals (debe de saber…)
👉 He trains every day, he must be in shape (debe de estar en forma)
•We use can’t + infinitive (without to) if we are sure that something is not true
👉 This jacket can’t belong to Mike. It’s too small (no puede pertenecer a…)
👉 He wears a chef hat. He can’t be a policeman (no puede ser…)
Certainty about the past
•We use must + have + past participle if we are sure something was true
👉 Sheila got a tan. She must have spent a lot of time in the sun lately (debe de haber pasado…)
👉 There was one banana left, but now it’s gone. My husband must have eaten it (debe de haberla comido)
•We use can’t + have + past participle if we are sure something was not true
👉 Martha can’t have taken your notebook; she wasn’t even in class yesterday (no puede haberlo cogido)
👉 He can’t have forgotten that there was an exam, because I told him 5 min before (no puede haber olvidado…)