GV

Modal Verbs for Deduction

Making logical conclusions: must be, can't be, might be, could be

intermediate

When to Use Modal Verbs for Deduction

  • •Must be: almost certain (positive). She must be tired. (I'm sure)
  • •Can't be: almost certain (negative). He can't be at home. (I'm sure he isn't)
  • •Might/May be: possible. It might be true. (Maybe)
  • •Could be: possible. She could be right. (Perhaps)
  • •Past deduction: must have been, can't have been, might have been.

How to Form Modal Verbs for Deduction

Affirmative (+)

Subject + modal + be + adjective/noun; Subject + modal + be + -ing

Negative (-)

Can't/couldn't + be (not mustn't for deduction)

Question (?)

Could it be...? Might she be...?

💡 Mustn't = prohibition, not negative deduction. Use can't for negative certainty.

Examples of Modal Verbs for Deduction

✓She must be sleeping. (I'm sure)

✗She must sleep. (wrong meaning)

Must be + -ing for present deduction.

✓He can't be the thief. (I'm sure he isn't)

✗He mustn't be the thief.

Can't for negative deduction; mustn't = prohibition.

✓They might have left already.

✗They might left already.

Modal + have + past participle for past.

Common Mistakes with Modal Verbs for Deduction

💡 Practice Tips for Modal Verbs for Deduction

  • • Certain: must be (positive), can't be (negative).
  • • Possible: might/may/could be.
  • • Past: must have been, can't have been, might have been.
  • • Mustn't ≠ negative deduction (it means prohibition).
Quick Quiz
  1. 1.The lights are on, so they   be home.
  2. 2.He's only twenty — he   be a grandfather!
  3. 3.She didn't answer. She   have been asleep.
  4. Question 4: Your bag is heavy! It blank (be) full of books.
    4.Your bag is heavy! It (be) full of books.
  5. Question 5: He's not answering his phone. He blank (be) in a meeting.
    5.He's not answering his phone. He (be) in a meeting.
  6. Question 6: She speaks perfect Italian — she blank (live) in Rome for years.
    6.She speaks perfect Italian — she (live) in Rome for years.
  7. Question 7: I saw John this morning, so he blank (not be) on holiday this week.
    7.I saw John this morning, so he (not be) on holiday this week.
  8. Question 8: The roads are wet — it blank (rain) during the night.
    8.The roads are wet — it (rain) during the night.
  9. Question 9: I'm not sure, but they blank (leave) already — their car is gone.
    9.I'm not sure, but they (leave) already — their car is gone.
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