In English, there are several modal verbs of deduction. They allow us to guess if something is true based on the information available. Four very important ones to know are must, can’t, may (not), and might (not).[1]https://learnenglish.britishcouncil.org/grammar/intermediate-to-upper-intermediate/modals-deductions-about-the-present
Must
You use the word “must” when you are 100% (or nearly) positive that something is true.[2]https://smitkit10-11.weebly.com/deduction.html
For example:
- You must be tired after that workout. (I am very sure that you are tired)
- I don’t have my phone. It must be on my desk. (I am sure that the phone is on my desk)
- She must study for the test if she wants to do well. (I am very sure that if she wants to pass the test it is required that she study)
May (not) & Might (not)
You use the words “may (not)” or “might (not)” if you are 50% sure of a fact.
For example:
- They are not here yet. They might be stuck in track. (I am not exactly sure why they aren’t here)
- Frank may be able to help us with the problem. (There is a 50% chance that Frank can help)
- The airline told us that the flight might be delayed due to weather. (There is a 50% chance that the flight gets delayed due to weather)
Difference between Might and May
May is used for situations that could be factual or could be possible.
Might is used for speculative situations that didn’t actually happen.[3]https://writingexplained.org/may-vs-might-difference
Can’t
You use the word “can’t” when you are 100% (or nearly) sure that something is false.[4]https://www.test-english.com/explanation/b1/modal-verbs-deduction-must-might-cant/
For example:
- Sorry, I can’t do that. It’s against the law. (I am 100% unable to do the task)
- You can’t stay in the house while they paint it. The fumes are toxic. (You are 100% not allowed to be in the house)
- I can’t drive today. I just had an eye test and can’t see. (Due to the eye test, I am 100% unable to drive)
Example Exercises
References