Grammar · Free lesson
Must be or can’t be: deductions
A delivery arrives with your name and the order number you expected. You draw a conclusion from those clues.
Professor Scott MendozaThe short answer
Use must be when the evidence makes you feel sure something is true. Use can’t be when it makes you feel sure something is not possible. Both express a conclusion, not an instruction.
Connect the conclusion to a clue.
“This must be my order” means that the clues strongly support that conclusion. It does not mean the package has an obligation. “This can’t be my order” rejects that possibility. After must or can’t, use the base form: be, not is or to be.
Compare the examples
The name and order number match. This must be my order.
I ordered a small notebook. This huge box can’t be my notebook.
There is no label. This might be my order.
Must be is the speaker’s judgment. Even a confident conclusion can turn out to be wrong.
Build the meaning step by step
State the clue
The order number matches.
Draw the conclusion
This must be my package.
Try it now
Read the clue
Read all answers and explanations
The sender and order number match. Express a strong positive conclusion.
This must be my package.Must + be gives a strong positive deduction. Can’t be rejects it; must is uses the wrong verb form.
Mina is beside you. You feel sure the person calling from another city is not Mina. Choose that conclusion.
That can’t be Mina.Can’t be expresses the requested strong rejection. Must be would support the opposite conclusion.
You have only a weak clue. Which sentence leaves the possibility open without sounding sure?
It might be the right address.Might be leaves room for doubt. Must be and can’t be express stronger judgments in these sentences.
Put it into practice
Repair the deduction
The parcel has your name and exact order number. Keep the strong positive conclusion and fix the form.
This must to be my parcel.
See a model answer
This must be my parcel.
Keep the judgment; repair only the verb pattern.
A little more clarity
Common questions
A few useful details before you move on.
Does must always mean an obligation?
No. Context matters. A rule can use must for an obligation; a conclusion from clues can use must for a deduction.
Visitors must sign in. / That must be the visitor we expected.
Why practice can’t be rather than mustn’t be here?
Can’t be clearly expresses a confident rejection of a possibility. Must not can express a prohibition and can have other readings in context, so do not treat it as an automatic negative version of every deduction.
That can’t be my seat; my ticket says row 12.
Take it into your day
Strong support: must be. Strong rejection: can’t be.
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