Overview
Express degrees of probability using will, should, may, might, and could.
Certainty Scale
- 100%will / must — near certain: She must be at home.
- ~70%should / ought to — expected: He should arrive by noon.
- ~50%may / might / could — possible: It might rain later.
- 0%can't / couldn't — impossible: That can't be true.
Past Probability
- must have + PP: She must have forgotten. (near certain)
- might/may have + PP: He might have missed the bus.
- can't / couldn't have + PP: She can't have finished already.
- should have + PP: They should have arrived by now.
Common Mistakes
- ❌ It musts be → ✅ It must be (modal never takes -s)
- ❌ She might to be late → ✅ She might be late
- ❌ He couldn't finish it (ability) vs can't have finished (deduction)
When to use
Deduction
The lights are off — they must have gone out.
Prediction
It might snow tonight — bring a coat.
Impossibility
That can't be right — I double-checked.
Expectation
She should be here by now.