How Do We Use Must, Might and Can't for Guessing?
You know must as "have to": You must wear a helmet. The same word has a second job. Before a guess, it means "I'm almost sure".
ALMOST SURE: YESmust + base verb
How sure is the speaker?
You've worked all day. You must be tired.
NOT SUREmight / may / could + base verb
He's not at his desk. He might be in a meeting.
ALMOST SURE: NOcan't + base verb
That can't be Minh. He's in Japan this week.
These are guesses from what you see or know, not rules.
TRY IT
- Question 1
"The restaurant is always full. The food blank be really good."
The forms
ALMOST SURE
Smustbase verb
Your sister must know a lot about cars.
No to after must.
NOT SURE
Smight / may / couldbase verb
The shop may be closed today. It's a holiday.
All three mean perhaps.
PERHAPS NOT
Smight not / may notbase verb
Nam might not come tonight. He's feeling ill.
Could not does not mean perhaps not.
ALMOST SURE NOT
Scan'tbase verb
You can't be serious!
The opposite of this must is can't.
HAPPENING NOW
Smust / might beverb + -ing
Listen! The neighbours must be watching football.
A guess about right now.
Might and may also work for the future: I might go to Đà Lạt next weekend.
TRY IT
- Question 1
"I don't know where my keys are. They blank be in the car, or maybe in my bag."
When to use them
The doorbell rings
Someone's at the door. That must be the delivery man.
You are waiting for a parcel.
Plans that aren't fixed
We might go to the beach on Sunday.
Someone asks about your weekend. You haven't decided.
Often used with: later · tonight · next week
Mustn't or Can't?
| MUSTN'T | CAN'T (GUESS) | |
|---|---|---|
| Meaning | not allowed | almost sure it is not true |
| Example | You mustn't smoke in here. | This can't be your bag. Yours is black. |
For "I'm sure that's not true", say can't, not mustn't.
TRY IT
- Question 1
"She blank be at work today. I just saw her at the market."
Common mistakes
Incorrect:He mustn't be at home. The lights are off.
Correct:He can't be at home. The lights are off.
Mustn't means not allowed. For a sure no, use can't.
Incorrect:It can rain this afternoon.
Correct:It might rain this afternoon.
For one possible event, use might or may.
Incorrect:She must to be at the airport by now.
Correct:She must be at the airport by now.
No to after must.
Incorrect:They must waiting for us.
Correct:They must be waiting for us.
A guess about now needs be + -ing.
Quick check · 5 questions
Choose the correct word:
Hoa has been studying since 7 a.m. She must be exhausted.
- A. must — Correct
- B. can't
- C. mustn't
Answer: A. must
Hours of study since 7 a.m. make it very likely.
Choose the correct word:
This can't be Lan's phone. Hers is pink, and this one is black.
- A. mustn't
- B. can't — Correct
- C. might
Answer: B. can't
Mustn't means "not allowed". It is not for guessing.
Choose the correct word:
Don't wait for me for dinner. I might be late. The traffic is terrible.
- A. might — Correct
- B. can
- C. must
Answer: A. might
Can is not used for a guess about one future event.
Choose the correct words:
Nobody is answering the phone at the office. They must be having a meeting.
- A. must be having — Correct
- B. must having
- C. must are having
Answer: A. must be having
Be stays in its base form after must.
Which sentence means "perhaps he doesn't know"?
He might not know.
- A. He might not know. — Correct
- B. He can't know.
- C. He mustn't know.
Answer: A. He might not know.
Can't know is much stronger. Mustn't know means he is not allowed to know.
See whether it really stuck.
Start the quiz · 14 questions