Modals show how sure you are that something is true. Must: you're almost certain (She must be tired.). Should: you expect it (The parcel should arrive today.). Might / may / could: it's possible (He might be at home.). Can't: you're sure it isn't true (That can't be Ana. She's in Spain.).
She's been working all day. She must be tired.
A gauge from impossible to almost certain. Must sits near the top: you're not 100% sure, but the evidence is strong.
| must | Almost certain, based on evidence: It must be late. Everyone's gone home. |
|---|---|
| should | Expected, if everything goes normally: The train should be here soon. |
| might / may / could | Possible, but you're not sure: It might rain later. |
| can't / couldn't | Almost certainly not true: That can't be right. |
For certainty, the opposite of must is can't, not mustn't: He can't be at home. His car isn't there.
You've been travelling all day. You must be exhausted.
You don't know for sure, but the evidence points one way.
Take an umbrella. It might rain later.
Maybe yes, maybe no. Might is the most common in speech.
The train should arrive at 10.
It's what you expect if nothing goes wrong.
That can't be right. Check the numbers again.
The evidence tells you it's not true.
Common expressions worth learning as they are.
Her car isn't here. She _____ at work.
How sure, how necessary, how polite.
Paste an email or an essay. Clarigram marks the mistakes you actually make, shows the rule behind each one as a picture, and builds your practice from them.
For possibility they're almost the same. Might is a little less sure and more common in speech: It might rain. May sounds slightly more formal.
For possibility, yes: It could be true means It might be true. But couldn't means impossible: It couldn't be true.