Deduction: Must, Can't and Might helps you communicate about strong positive, strong negative and tentative deduction. Start with the meaning, notice the pattern, and then use it in complete sentences.
By the end of this lesson, you can
- I can make logical deductions about present situations using evidence.
- Recognise and build the form: must/can't/might + base verb.
- Use the pattern accurately in a meaningful sentence.
Meaning and use
Use Deduction: Must, Can't and Might for strong positive, strong negative and tentative deduction. Always read the whole situation before choosing the form: grammar expresses a meaning, not only a pattern.
The lights are on; someone must be home.
Use 'must' for a strong logical conclusion based on evidence.
He is in Paris, so that man can't be him.
Use 'can't' when evidence makes a conclusion impossible.
I am not sure, but this might be the right address.
Use 'might' for a tentative deduction.
must/can't/might + base verbKeep this pattern visible while you study the examples. Then cover it and rebuild one example from memory.
Key rules to notice
These rules explain why the form is correct. Focus on the subject, auxiliary, verb form, word order and time or meaning clues.
- Use 'must' for a strong logical conclusion based on evidence.
- Use 'can't' when evidence makes a conclusion impossible.
- Use 'might' for a tentative deduction.
- Her detailed knowledge supports a strong positive deduction.
- The smell is evidence that the food cannot be fresh.
Common mistakes
Not:The lights are on; someone can't be home.
Use:The lights are on; someone must be home.
Use 'must' for a strong logical conclusion based on evidence.
Not:He is in Paris, so that man must be him.
Use:He is in Paris, so that man can't be him.
Use 'can't' when evidence makes a conclusion impossible.
Not:I am not sure, but this must be the right address.
Use:I am not sure, but this might be the right address.
Use 'might' for a tentative deduction.
Check before you practise
Complete each sentence mentally, then reveal the answer.
She knows every detail; she ____ know the team well.
must
Her detailed knowledge supports a strong positive deduction.
This ____ be fresh; it smells terrible.
can't
The smell is evidence that the food cannot be fresh.
They haven't replied; they ____ be busy.
might
No reply gives only a possible, not certain, explanation.