Modal verbs of certainty

1. Modals of certainty

2. The modals of certainty in English

Modals of certainty are used to make deductions or assumptions. We will choose the modal (must, can't, may, might and could) according to the degree of certainty we have about what we are saying.

3. Structure of the modals of certainty

• When the deduction is about habitual and everyday situations or the main verb is a stative verb the modal verb is followed by an infinitive. The structure is as follows:
  

  	Form	Examples
  MUST	Subject + must + infinitive	He is a biologist, he must know a lot about animals.
  CAN'T	Subject + can't + infinitive	This jacket can't belong to Mike. It's too small.
  MAY MIGHT COULD	Subject + may/might + not + infinitive	I wouldn't go to his house. He might not be there.

  ¹We have not put the negative form of could because when it acts as a modal of certainty, it is only used in affirmative sentences.

  - They are very famous swimmers.

  - They must train lots of hours every day.
• However, we will use the modals with the verb to be followed by a gerund when dealing with deductions about temporary situations and actions happening at the moment of speaking.
  

  	Form	Examples
  MUST	Subject + must + be + gerund	Paula is in Hawaii. Now she must be lying on the beach.
  CAN'T	Subject + can't + be + gerund	They could be talking about the weekend.
  MAY MIGHT COULD	Subject + may/might + not + gerund	He might not be lying.

  ¹No hemos puesto la forma negativa de could porque cuando actúa como modal de certeza, solo se utiliza en oraciones afirmativas.

  - I don't know where he is going.

  - He might/may/could be going home.

4. Uses of the modals of certainty

• We use can't when we are quite sure that something is impossible.
  
  - Paula is smoking.

  - She can't be smoking, she gave it up.
• We use must when we are quite sure that something is true.
  
  - Where is your sister?

  - She must be lying on the sofa.
• Might, may and could are used about situations in the present when the speaker is unsure of the deduction or assumption he or she is making because of lack of information or because it is not evident.
  
  - What language are they speaking?

  - I don't know. It might/may/could be Swedish.
• Might, may and could are also used when the speaker is unsure of an assumption he or she is making about the future.
  
  I may tell you the secret.

  It  might not be sunny tomorrow.

Remember!