Study materials
You walk up to a house in the evening and see light in the windows. You do not know the fact, but you draw the conclusion at once.
The lights are on. Eva must be home.
Her car isn't here. She can't be home.
This is not an order or a ban, though the words are the same as in the topic about obligation. Here they measure something else: how sure you are of your own conclusion.
The scale of certainty
There are few words, and they line up from near certainty to logical impossibility.
| Word | How sure you are |
|---|---|
| must | almost certain it is so |
| may, might, could | perhaps, perhaps not |
| can't | almost certain it is not |
She must know the answer.
She might know the answer.
She can't know the answer.
The negative is can't, not mustn't
Here is the most important detail of the topic. You would expect the opposite of must to be mustn't, but in the sense of deduction it is not.
He is in Rome, so he can't be at the office.
You mustn't park here.
So remember the pair: must for a positive conclusion, can't for a negative one. Mustn't is not used in this sense at all.
What exactly you are guessing about
A base form always follows the modal. But the sentence itself can be built differently, depending on whether you mean a state or a process.
They may be tired.
They may be waiting outside.
The modal does not change with the person and does not take to. She might know, not she might knows and not she might to know.
The difference between may, might and could
In the sense of a guess these three are nearly interchangeable. There is a shade of difference, but a thin one: might and could sound slightly more cautious than may.
The keys could be in the car.
The keys might be in the car.
None of the three would count as a mistake, so the choice is not worth agonising over.
The mistakes everyone makes
The first is taking mustn't for a negative conclusion.
He mustn't be at home, the lights are off. → He can't be at home, the lights are off.
The second is adding to after the modal.
She must to be tired. → She must be tired.
The third is confusing a conclusion with a demand.
He must be forty.
He is probably about forty. A guess, not an order to be forty.
All three are caught by one question. Am I demanding something or guessing something? If guessing, the negative is can't.
Try it right now
Look out of the window or at a person nearby and draw three conclusions in English: one confident, one uncertain, and one about what definitely cannot be the case.
Check that the last sentence has can't in it and not mustn't.