Modal verbs of deduction: Must, may, might, could, can’t

Modal verbs of deduction: Grammar chart

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May, might

We use may and might to talk about things that are possibly true, but we don’t know for sure.

• He’s more than 2 meters tall. He might be a basketball player. (=perhaps he is)
• He says Betty is his friend, but I think she may be his girlfriend.

We use may not or might not to talk about things that are possibly not true, but we don’t know for sure.

• You should call her. She might not know where you are. (=perhaps she doesn’t know)

Don’t use can for deduction

We don’t use can as a modal of deduction.

• He can be at home now.
• He might/may be at home now.

Must, can’t

We use must when we are sure, or quite sure, that something is true.

• You must be tired after the long journey. (=I’m sure you are tired)
• I’m sure I had the keys when I left. They must be in the car.

❗ But we use can’t (NOT mustn’t) when we are sure, or quite sure, that something is not true.

• We’ve been walking for hours. It mustn’t be far from here.
• We’ve been walking for hours. It can’t be far from here. 
• They’ve lived here only for a couple of months. They can’t know many people.

+ be + -ing

After may, might, must or can’t, we can use be + -ing, when we are talking about actions in progress.

• They’ve gone to Ibiza, and right now, they must be having a great time.
• Call him. He might be waiting for us.