Must and Can’t – Modal Verbs Practice

Grammar Summary

Must and can’t are modal verbs used to express certainty and logical deduction.

Must

Use must to say that you believe something is certain:

• “You’ve been travelling all day. You must be tired.” (logical conclusion)
• “Louise must get very bored in her job. She does the same thing every day.” (deduction)

Can’t

Use can’t to say that you believe something is not possible:

• “You’ve just had lunch. You can’t be hungry already.” (logical impossibility)
• “They haven’t lived here for very long. They can’t know many people.” (deduction)

Structure for Present Situations:

Subject	Modal	Forms
I/you/he etc.	must can’t	be (tired/hungry/at work etc.) be (doing/going/joking etc.) do/get/know/have etc.

For Past Situations:

We use must have (done) and can’t have (done) or couldn’t have (done):

• “There’s nobody at home. They must have gone out.” (certainty about past)
• “Sarah hasn’t contacted me. She can’t have got my message.” (impossibility about past)

Subject	Modal	Auxiliary	Forms
I/you/he etc.	must can’t	have	been (asleep/at work etc.) been (doing/looking etc.) gone/got/known etc.