Modal 'MUST & CAN NOT'


We use Must to express that we are sure that something is true:
-   Kevin has been working all day. He must be tired.
-   Sarah does the same thing every day. She must get bored in her job.
-   Andrew’s house is near the railway station. It must be very noisy.

On the other hand, in order to say the opposite, that something is impossible to happen, we use Can Not/Can’t:
-   Kevin has just had breakfast. He can’t be hungry already.
-   Sarah is not good at Math. She didn’t study last night. She can’t be passing the exam.
-   They moved here one month ago. They can’t know many people.

The Past form of Must and Can’t are Must Have + V3 and Can’t Have + V3 :
-   I knocked the door many times but nobody opened the door. Kevin must have been asleep.
-   Sarah has lost her wallet. She must have dropped it somewhere.
-   Jason walked past me without speaking. She can’t have seen me.
-   Jason has already resigned from his work. He can’t have been enjoying working there.
-   jason has already resigned from his work. He can’t have enjoyed working there.
After HAVE we may use either BEEN + V ing or just V3 (Past Participle)

We may use Couldn’t Have to replace Can’t Have:
-   She couldn’t have seen me.
-   He couldn’t have been enjoying working there.

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