1. MODALS OF POSSIBILITY

2. IMAGINE YOU ARE AT HOME, RELAXING YOURSELF... WHEN SUDDENLY, THE DOORBELL RINGS

3. You can’t see who is at the door

5. But what’s the difference?

7. THAT can’t BE THE POSTMAN This means you are CERTAIN that it is not the postman.

9. THAT could BE THE POSTMAN This means you are also NOT CERTAIN it is the postman but it is possible or probable.