[A] CHILD: Can I have some sweets? I’m hungry. MOTHER: You can’t/couldn’t be hungry. You’ve just had dinner; ANN (looking through binoculars): An aeroplane is pulling up people from the boat. TOM: It can’t/couldn’t be an aeroplane. It must be a helicopter; He says he’s still reading «The Turn of the Screw.» – He can’t/couldn’t still be reading it. I lent it to him ages ago and it’s quite a short book. [B] A man answered the phone. I suppose it was her husband. – It can’t/couldn’t have been her husband. He’s been dead for ages; I took a Circle Line train to St Paul’s. – You can’t/couldn’t have taken the Circle Line. It doesn’t go through St Paul’s. You must have been on the Central Line. She said I couldn’t have come on the Circle Line; He said it couldn’t be an aeroplane.
[[A] Las deducciones negativas en relación con un acontecimiento presente se pueden expresar por medio de can’t/couldn’t seguidos del infinitivo de presente del verbo be o del infinitivo continuo de cualquier verbo. [B] Las deducciones negativas en relación con un acontecimiento pasado se expresan por medio de can’t/couldn’t + el infinitivo de perfecto o el infinitivo de perfecto continuo de cualquier verbo. El uso de couldn’t es obligatorio cuando la deducción está hecha en el pasado o aparece introducida por un verbo en pasado; si el caso es otro, se pueden usar indistintamente can’t o couldn’t.]
A Practical English Grammar (A.J. Thomson & A.V. Martinet) 