Modals for Deduction

Use and Form:  

The following modals can be used to make guesses about a present situation.

must + infinitive
Use this when you make a guess and you are almost certain that your guess is correct.
‘Where’s John?’
‘He’s not here. He must be in the bathroom.’

may + infinitive 
might + infinitive 
could + infinitive 
Use this when you make a guess but you are only suggesting one possibility. You are not certain you are correct.
‘Where’s John?’
‘He’s not here. He may be in the bathroom, or he might be in the kitchen, or he could be outside.’

may not + infinitive 
might not + infinitive 
Use this when you make a guess about what is not true, but you are only suggesting one possibility. You are not certain you are correct.
‘Where’s John?’
‘He’s not here. He may not be at work today.’

NOTE: Do not use could not here.

can’t + infinitive
Use this when you make a guess about what is not true, and you are almost certain that your guess is correct.
‘Where’s John? Is he in the kitchen?’
‘No, he can’t be. I was in there a minute ago.’

NOTE: you cannot use: mustn’t + infinitive to make deductions about what is not true.
 

Common Mistakes:
1. Many students do not take the opportunity to use these structures when they can.
Maybe your bag is in the classroom.        =>           Your bag might be in the classroom.