One of the functions of modal verbs is to express speculation.
Depending on the degree of probability of our deduction, we use different verbs.
- The verb “must” is an almost absolute certainty:
The phone’s ringing. It must be Paul.
- Its opposite is “can’t” and “couldn’t”, meaning that a situation does not happen.
The phone’s ringing, but it can’t be Paul. He’s still at work.
- The verbs “may/might/could” are used to express possibilities, but could is a little weaker in its expression:
The phone’s ringing. It may be Paul.
The phone’s ringing. It might be Paul.
The phone’s ringing. It could be Paul.
|He doesn’t know.|
|unlikely||can’t/ couldn’t||He can’t know.|
|50% probability||could||He could know.|
|quite likely||may/ might||He may know.|
|very likely||must||He must know.|
|100% probability||Simple Present||He knows.|