Unit 7.1

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Must and Cannot: Modals of Deduction


A modal verb is a type of auxiliary (helping) verb that has no meaning on its own but it modifies the main verb, changes its meaning and gives more details about the action.

Must and cannot are modal verbs that can be used even to express deductions.


The modal verbs must and cannot are used as auxiliary verbs and their structures are affirmative and negative.


Its structure, in the affirmative form, is:
Subject + must + verb + …


Its full structure, in the negative form, is:
Subject + cannot + verb + …

The negative short form is: Can’t
Subject + can’t + verb + …




  • Your forehead is so hot! You must have a fever.
  • She’s shivering. She must be cold.
  • I don’t see him anywhere, he must be late. 


  • It cannot be true that she’s getting married. 
  • He is late, he can’t come in.
  • They aren’t answering the phone. They can’t be home.


We use must and cannot for deductions if we have strong evidence.

  • We use must in affirmative clauses, when we are sure about something and we have strong evidence for what is happening;
  • We use cannot in negative clauses when we believe that something is not true or possible to happen.


We can use the modal verbs must and cannot to deduce certain things due to strong evidence.

Must is used in affirmative clauses, when we are sure about something, whereas cannot is used in negative clauses, when we believe that something is not true or possible to happen.

We can use it in affirmative and negative forms:

  • affirmative: we start with the subject followed by must and a verb.
  • negative: we start with the subject followed by cannot and a verb.

For example:
“He has two private jets, he must be rich.” = We are sure that he is rich, we have enough evidence to prove it.
“The show starts in 5 minutes. They can’t be on time.” = It is impossible for them to be on time.
“He has two private jets. He has to be rich!” = We use have to when something is reasonably expected or a certainty.

Let’s revise this content within the {Form} section. Take a look at the {Example} section that shows its use within a context.