1. Modals of Certainty and Uncertainty Bryan Lopez
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2. A: Do you know where John is? B: No, I don't. He might be on a break.

3. Certainty

4. If we are less than 100% certain about a situation, we can use modal verbs to show our level of certainty:

5. 100% certain
He is on a break.

6. Must means you are very sure, but not 100%.
*Although not modal verbs, have to and have got to can be used as the same meaning of must, especially in American English.
Almost
100%
certain
He must be on a break.
He has to be on a break.
He's got to be on a break.
Must means you are very sure, but not 100%.

7. Uncertain
He may be on a break.
He might be on a break.
He could be on a break.
May, might, and could are used to say that you are unsure.

8. Examples
He must be hungry. He hasn't eaten anything all day.
She has to be here. That's her bag and coat.
She has to go home early. She might have to do her homework still.

9. In negative situations, we say the following:

10. 100% certain
He isn't here.

11. Almost 100% certain
He can't be here.
Can't is commonly used to express a strong certainty that something is impossible.

12. Uncertain
He may not be here.
He might not be here.
May not and might not express uncertainty.
Notice could is not used for this meaning.

13. Examples
She joined the school this morning, so she can't know the rules of the school yet.
You said Joe was Canadian, but he speaks English with an Irish accent. He can't be Canadian.
Jose may not understand what you are saying. His English is not as good as mine.