Modal Verbs for Deduction

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

intermediate

When to Use

•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

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

✓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

💡 Practice Tips

• 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).

modal-probability modal-necessity past-perfect 📝 modals-intermediate 📝 modal-deduction-quiz

Modal Verbs for Deduction

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

intermediate

When to Use

•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

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

✓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

💡 Practice Tips

• 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).

modal-probability modal-necessity past-perfect 📝 modals-intermediate 📝 modal-deduction-quiz

Modal Verbs for Deduction

When to Use

How to Form

Examples

Common Mistakes

💡 Practice Tips

Related

Modal Verbs for Deduction

When to Use

How to Form

Examples

Common Mistakes

💡 Practice Tips

Related

When to Use

How to Form

Examples

Common Mistakes

✗He mustn't be at home.

✗She must be know the answer.

✗They can have arrived.

💡 Practice Tips

Related

When to Use

How to Form

Examples

Common Mistakes

✗He mustn't be at home.

✗She must be know the answer.

✗They can have arrived.

💡 Practice Tips

Related