Modals of deduction

Use must, might, may and cannot to express how certain you are about a present situation.

When do we use it?

Distinguish strong certainty, possibility and logical impossibility.

Form

Use must for a strong positive deduction, might/may/could for possibility and can’t for a strong negative deduction.

• She must be tired.
• He might be at home.
• That can’t be true.

Key contrast

Read the whole context and identify the time, meaning, noun type or sentence structure before choosing.

Common mistakes

• She must to be tired. → She must be tired.
• It can be impossible. → It can’t be true.

Remember

Use must, might, may and cannot to express how certain you are about a present situation.

Quick check

• Find the grammar clue.
• Check the subject and verb form.
• Confirm that the answer fits both structure and meaning.