Past modal verbs of deduction. For past deductions, use modal + have + past participle: must have for strong belief, might/could have for possibility and can't/couldn't have for impossibility.
The idea
Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Structure & examples
All the lights are off. They must be asleep.
That cannot be Lina; she is abroad.
Notice the pattern
These modals show how certain you are, not whether someone has permission. Must expresses a strong positive inference; cannot expresses a strong negative one; may, might and could express possibility. For a past inference, use modal + have + past participle.
She must be very busy.
They might be at lunch.
Important notes & common mistakes
Deduction is an inference rather than an obligation.
Before you practice
First choose a form from the options. Then produce it yourself using the word or structure in the instruction. Finally, build complete sentences. Check the explanation after each answer and revisit any mistakes in your review list.
Extended topic: Past modal verbs of deduction
For past deductions, use modal + have + past participle: must have for strong belief, might/could have for possibility and can't/couldn't have for impossibility. Distinguish could have as uncertain deduction from unrealised opportunity by context.
Examples in context
Notice how each form fits its sentence. Read the context before choosing a form.
The lights are on. They must have returned.
He can't have seen it; he wasn't there.
I'm unsure; she might have missed the bus.
They may have left already, but I don't know.
The road is wet. It must have rained.
Your progress
Ready to move on?
Mark this lesson as studied when you have finished the explanation.
Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
This ___ be my coat; mine is blue and this one is red.
Your answer: Unanswered
Correct answer: cannot
Completed sentence: This cannot be my coat; mine is blue and this one is red. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Her car is outside and the lights are on. She ___ be at home.
Your answer: Unanswered
Correct answer: must
Completed sentence: Her car is outside and the lights are on. She must be at home. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
It is possible that Jo is busy. She ___ be in a meeting.
Your answer: Unanswered
Correct answer: might
Completed sentence: It is possible that Jo is busy. She might be in a meeting. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
He has not eaten all day. He ___ be hungry.
Your answer: Unanswered
Correct answer: must
Completed sentence: He has not eaten all day. He must be hungry. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Question 1 of 50% complete
All the lights are off. They ___ be asleep.
Not quite. Keep learning.
Correct answer: must
Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: cannot
Completed sentence: This cannot be my coat; mine is blue and this one is red. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: must
Completed sentence: Her car is outside and the lights are on. She must be at home. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: might
Completed sentence: It is possible that Jo is busy. She might be in a meeting. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: must
Completed sentence: He has not eaten all day. He must be hungry. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: must
Completed sentence: Use must for a strong deduction: You ran ten kilometres. You must be tired. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: cannot
Completed sentence: Use cannot for a negative deduction: That cannot be Paul; Paul is much taller. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: might
Completed sentence: Use might for a possibility: I am not sure, but they might be away. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: must
Completed sentence: Use must for a strong deduction: The ground is wet. It must have rained. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: cannot
Completed sentence: Use cannot for a negative deduction: This total cannot be right; it is negative. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction.
Not quite. Keep learning.
Correct answer: That cannot be Lina; she is abroad
Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction. Deduction is an inference rather than an obligation.
Not quite. Keep learning.
Correct answer: She must be very busy
She must be very busy. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction. Deduction is an inference rather than an obligation.
Not quite. Keep learning.
Correct answer: They might be at lunch
They might be at lunch. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction. Deduction is an inference rather than an obligation.
Not quite. Keep learning.
Correct answer: That cannot be the right address
That cannot be the right address. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction. Deduction is an inference rather than an obligation.
Not quite. Keep learning.
Correct answer: He must have missed the bus
He must have missed the bus. Use must for a strong positive deduction, might for possibility, and cannot for a strong negative deduction. Deduction is an inference rather than an obligation.
Not quite. Keep learning.
Correct answer: returned
Completed sentence: The lights are on. They must have returned. For past deductions, use modal + have + past participle: must have for strong belief, might/could have for possibility and can't/couldn't have for impossibility. Distinguish could have as uncertain deduction from unrealised opportunity by context.
Not quite. Keep learning.
Correct answer: seen
Completed sentence: He can't have seen it; he wasn't there. For past deductions, use modal + have + past participle: must have for strong belief, might/could have for possibility and can't/couldn't have for impossibility. Distinguish could have as uncertain deduction from unrealised opportunity by context.
Not quite. Keep learning.
Correct answer: have
Completed sentence: I'm unsure; she might have missed the bus. For past deductions, use modal + have + past participle: must have for strong belief, might/could have for possibility and can't/couldn't have for impossibility. Distinguish could have as uncertain deduction from unrealised opportunity by context.
Not quite. Keep learning.
Correct answer: may
Completed sentence: They may have left already, but I don't know. For past deductions, use modal + have + past participle: must have for strong belief, might/could have for possibility and can't/couldn't have for impossibility. Distinguish could have as uncertain deduction from unrealised opportunity by context.
Not quite. Keep learning.
Correct answer: rained
Completed sentence: The road is wet. It must have rained. For past deductions, use modal + have + past participle: must have for strong belief, might/could have for possibility and can't/couldn't have for impossibility. Distinguish could have as uncertain deduction from unrealised opportunity by context.
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