028 : must and can’t

   A   

Study this example:

 

 

You can use must to say that you believe something is certain:

• You've been travelling all day. You must be tired. (Travelling is tiring and you've been travelling all day, so you must be tired.)
• 'Joe is a hard worker.'  'Joe? You must be joking. He doesn't do anything.'
• Louise must get very bored in her job. She does the same thing every day.
• I'm sure Sally gave me her address. I must have it somewhere.

 

You can use can't to say that you believe something is not possible:

• You've just had lunch. You can't be hungry already. (People are not normally hungry just after eating a meal. You've just eaten, so you can't be hungry.)
• They haven't lived here for very long. They can't know many people.

 

Study the structure:

I/you/he (etc.)

must can't

be (tired / hungry / at work etc.) be (doing / going / joking etc.) do / get / know / have etc.

 

 

   B   

For the past we use must have (done) and can't have (done).

 

Study this example:

 

Martin and Lucy are standing at the door of their friends' house.
They have rung the doorbell twice, but nobody has answered. Lucy says:

 

They must have gone out.

 

• ‘We used to live very near the motorway.’  ‘Did you? It must have been noisy.’
• ‘I've lost one of my gloves.’ ‘You must have dropped it somewhere.’
• Sarah hasn't contacted me. She can't have got my message.
• Tom walked into a wall. He can't have been looking where he was going.

 

 

Study the structure:

I/you/he (etc.)

must can't

have

been (asleep / at work etc.) been (doing / looking etc.) gone / got / known etc.

 

You can use couldn't have instead of can't have:

• Sarah couldn't have got my message.
• Tom couldn't have been looking where he was going.

 

 

 

 (American English -> Appendix 7) 

 

British speakers use can't to say they believe something is not probable: Sarah hasn't contacted me. She can't have got my message

American speakers use must not in this situation: Sarah hasn't contacted me. She must not have gotten my message.