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maxresdefault - Coggle Diagram

- - - - Use When You Are Really Sure About Something.
        
        He must have failed the exam.
        
        For the negative form, " can't " is used, not " mustn't ".
  - - - Use When You Think It Is Not True.
        
        She can't have been asleep.
        
        The Structure " Couldn't + Have + V3 " also means the same thing.
        
        She couldn't have been asleep.
- - - - I could have finished the report if I had stayed longer at the office. ( I didn’t stay longer at the office. I went home. )
        
        We don't use " Couldn't Have V3 " with this meaning because use to say it is not possible that something happened ( similar to Can't Have V3 )
      - - Why isn’t John at work yet?
        + I don’t know, he might have missed the train. He/She could also use the past modal verb form of “might” but ıt indicates a lower probability.
        
        Quinoa is a simple food that you might not have heard of.
      - - Why isn’t John at work yet?
        + I don’t know, he may have missed the train. ( He/She suggests that he is late because he missed the train (in the past) )
        
        Quinoa is a simple food that you may not have heard of.