Modal verbs do more than express obligation or permission. They also show how sure the speaker is about a fact or a prediction. Different modals carry different levels of confidence — from near-certainty to a vague guess.
The choice of modal tells the listener: I'm almost sure, I think so, or it's just a possibility. This lesson maps the full scale.
The certainty scale
From strongest to weakest confidence:
- must — almost certain (~95%): She must be at home — her car is outside.
- will — confident expectation (~90%): That will be James at the door.
- should — reasonable expectation (~80%): The letter should arrive by Friday.
- may — open possibility (~50%): He may know the answer.
- might / could — weaker possibility (~30–40%): It might rain later. / She could be stuck in traffic.
And on the negative side:
- can't / couldn't — almost certain it's NOT true (~95%): That can't be right.
- may not / might not — possibly not true (~50% or less): She may not come tonight.
Every modal in this list is followed by the base form of the verb — no to, no changes for third person.
Positive deduction
must — I'm almost sure
Use must when the evidence points strongly in one direction and there is no realistic alternative.
Must in this sense is not about rules or obligation — it is about logical certainty. The speaker is not giving an order; they are drawing a conclusion.
will — confident prediction
Will expresses strong expectation, often based on knowledge of a person or a routine rather than visible evidence.
The difference from must: must relies on evidence you can see or already know. Will relies on what you expect from the situation.
should — reasonable expectation
Should suggests that something is likely to be true if everything goes as planned. There is room for surprise, but the speaker expects a normal outcome.
Should works best for neutral or positive expectations. For negative situations, English speakers often prefer will: The motorway will be busy at rush hour sounds more natural than should be busy.
may — open possibility
May sits in the middle of the scale. The speaker considers something possible but has no strong reason to lean either way.
May is slightly more formal than might in everyday speech, but the difference in certainty is small.
might and could — weaker possibility
Might and could express a possibility the speaker is not especially confident about. It is more of a guess than a judgment.
Could sometimes carries a slight extra nuance of "I can imagine a scenario where..." rather than "I have some evidence that...". In practice, might and could are often interchangeable.
Negative deduction
can't and couldn't — almost certain it's NOT true
Can't is the negative mirror of must. The speaker looks at the evidence and concludes that something is essentially impossible.
Couldn't works the same way, sometimes with a slightly softer tone:
may not and might not — possibly not true
These express uncertainty in the negative direction. The speaker thinks something is possible but not certain.
An important distinction: may not and might not express uncertain negation. Mustn't, by contrast, is not used for negative deduction in English — it means prohibition (You mustn't park here). This is one of the most common mistakes at this level.
Present situations vs. future predictions
The same modals work for both present states and future events. The time frame comes from context, not from the modal itself.
Present — guessing what is true right now:
Future — predicting what will happen:
In both cases the structure is the same: modal + base verb. No extra marking for present vs. future.
How to choose the right modal
When guessing or predicting, ask yourself one question: how strong is my evidence?
If you are unsure which modal to use, might is almost always a safe choice — it signals possibility without overclaiming.
Common mistakes
Using mustn't for negative deduction
Mustn't means prohibition, not impossibility. For negative deduction, use can't or couldn't.
Confusing must (deduction) with must (obligation)
Context makes the difference. In deduction, must is about what the speaker believes, not what someone has to do.
Adding to after the modal
Using can for specific present possibility
Can expresses general possibility (Prices can be high in London), not a guess about a specific situation right now.
Wrong negative form of may
Modal verbs form negatives with not directly after the modal, never with do/does.