We use modal verbs to say how sure we are about something.
We use must when we feel sure that something is true because there’s very strong evidence.
- He must live near here because he comes to work on foot. (We don’t know where he lives but we’re sure it’s not far away.)
2 might, may, could
We use might, may or could to say that we think something is possible but we’re not sure.
- Did you hear that? I think there might be a burglar downstairs. (She’s not sure there’s a burglar but she thinks it’s possible.)
We use can’t when we feel sure something is not true.
- It can’t be a burglar. All the doors and windows are locked. (He doesn’t know it’s not a burglar but he feels sure it’s not.)
The modals mustn’t and can aren’t used for this purpose.
Let’s practise! What do you assume from the following images?