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Senior Member
Hi everybody!
Could you help me to feel the difference between can't and couldn't.

1. "Sarah couldn't have got my message. Otherwise she would have replied. "
Here we are speaking about the PAST. We are sure that Sarah didn't receive the message yesterday, 1 hour ago, 1 minute ago. So she hasn't recieved the message yet.

2. "He can't have recieved my letter because I only posted it yesterday."
Her we are also speaking about the PAST.

My question: why have "couldn't" been used in the 1-st case, and why has "can't" been used in the 2nd case. In both cases we speak about the PAS
Thank you
  • boozer

    Senior Member
    Indeed, we are talking about past events. However, the past is expressed by 'have received' and 'have got', not by the modal verb that precedes them. In this case 'could' is not the past-tense form of 'can'. It is a modal verb in its own right. You can put other modal verbs there and it will still be about the past when you use the perfect infinitive:
    She may have received the letter
    He might have been killed
    They must have arrived
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