subject + must/might/can't + be + complementUse this for nouns, adjectives, and place phrases: She must be tired. He can't be at home.
Modals for Deduction
B1Use must for a strong guess, might for a possible guess, and can't for an impossible one. These modals show what you conclude from clues now.
What you'll learn
- Choose must when the evidence makes a conclusion feel almost certain.
- Choose might when a conclusion is possible but not certain.
- Choose can't when the facts show a conclusion is impossible.
- Build be-sentences with modal + be + noun, adjective, or place.
- Keep the main verb in the base form after the modal.
Structure
subject + must/might/can't + base verb + complementAfter the modal, use the base verb with no to and no ending: They might know. She must live nearby.
Build a sentence
Subject
Modal
Predicate
Annamustbetired
Anna must be tired.
With adjectives, use modal + be + adjective.
When to use
People and identity
Guess who someone is from clues. That must be Maria. The man at the door might be the driver.
Place and situation
Guess where someone is or what situation is true. Lisa must be at work. Tom can't be in the office.
Possible reason
Offer one explanation when you are not sure. Anna might feel sick. The store might be closed for lunch.
Markers
looks likesoundsfrom the lightsI guessprobablythere's no way
In contrast
vs must-strong-obligation
Deduction must = your conclusion from evidence: She must be tired. Obligation must = a rule or duty: She must finish the report today.
vs may-might-possibility
Might for deduction gives a guess from clues in the situation. Basic possibility may/might can also talk about what is generally possible.
Common mistakes
Wrong
She must at work.
Correct
She must be at work.
Use be after the modal before a place phrase or adjective.
Wrong
He might to know the answer.
Correct
He might know the answer.
After might, use the base verb without to.
Wrong
She must knows him.
Correct
She must know him.
After a modal, the main verb stays in the base form.
Wrong
I can't find Anna. She must be at the gym, but I'm not sure.
Correct
I can't find Anna. She might be at the gym.
If you are not sure and only have one possible explanation, use might, not must.
Wrong
It's midnight and Tom is still online. Tom must finish his report now.
Correct
It's midnight and Tom is still online. Tom must be working.
The second sentence is a conclusion from evidence, not an order.
Common misconceptions
❝ Must always means a rule or duty. ❞
Must can also show a strong conclusion from clues: The door is open. They must be home.
❝ Can't only talks about ability. ❞
Can't can also mean logical impossibility: He can't be asleep — he's talking on the phone.
Skills in this rule (7)
MUST_FOR_STRONG_GUESSw5Use must for a strong logical guess
Use must when the evidence makes you feel almost sure something is true now. The speaker is not ordering anyone; they are drawing a conclusion.
MIGHT_FOR_POSSIBLE_GUESSw5Use might for a possible guess
Use might when something is possible but you are not sure. It shows one possible explanation, not certainty.
CANT_FOR_IMPOSSIBLE_GUESSw5Use can't when something is impossible
Use can't when the facts show a conclusion is not possible. It expresses logical impossibility, not lack of ability.
MODAL_PLUS_BE_NOUN_ADJw4Build deduction with modal + be + noun or adjective
With identities, places, and adjectives, use must/might/can't + be. Common patterns are be late, be tired, be Maria, be at work.
MODAL_PLUS_BASE_VERBw5Use the base verb after must, might, and can't
After these modals, the next verb stays in the base form. Do not add -s, -ed, or to.
PICK_EVIDENCE_MARKERSw3Recognize clues that lead to a guess
Words and phrases like look, sound, probably, I guess, and from the lights signal a conclusion from evidence. They often appear with must, might, or can't.
TELL_GUESS_FROM_RULEw4Tell a logical guess from a rule or duty
In deduction, must is the speaker's conclusion about what is true. In obligation, must tells someone what to do.