Overview
Speculation and deduction use modal verbs to express degrees of certainty about present and past situations. Must (strong certainty): She must be at home. Might/could (possibility): He might have missed the train. Can't (strong negative certainty): That can't be right.
Present Deduction
- Certain+She must be exhausted — she worked a 16-hour shift.
- Certain–That can't be true — I checked the figures myself.
- PossibleHe might / may / could be in a meeting right now.
- ExpectedShe should be there by now — she left two hours ago.
Past Deduction
- Past certain+She must have missed the announcement.
- Past certain–He can't have finished — he only started an hour ago.
- Past possibleThey might / may have left before we arrived.
- Past expectedThe report should have been filed by noon.
Certainty Gradient
- 100% deduction: must / can't (logical necessity or impossibility)
- ~70% expectation: should / ought to
- ~50% speculation: may / might / could
- ❌ must be knowing → ✅ must know (stative verb = no -ing)
When to use
Detective reasoning
The door is locked from inside — someone must still be in there.
Medical diagnosis
These symptoms could indicate a viral infection.
Academic analysis
The anomaly may have resulted from measurement error.
News commentary
The minister's silence can't have been coincidental.