Study materials
Three sentences about the same person. The fact is identical, and your confidence differs each time.
She must be at home.
She should be at home.
She might be at home.
The modals here are not about permission or obligation. They measure how far you stand behind what you say.
A scale, not a list
Modal words are not interchangeable synonyms. They sit on a scale, and each marks its own level of certainty.
| Level | Forms | Example |
|---|---|---|
| near certainty | must, will | She must be home by now. |
| high probability | should, ought to | The parcel should arrive today. |
| open possibility | may, might, could | He might know. |
| logical impossibility | can't, couldn't | That can't be right. |
The form will in this sense often surprises learners: it reports not the future but a confident inference. That will be the courier means "that is probably the courier".
Where the certainty came from
At this level not only the degree matters but the source. The same confidence may rest on evidence, on experience or on somebody else's words, and the form gives that away.
The lights are on, so she must be home.
She should be home by six.
She is said to be home.
She seems to be home.
Should in this sense carries a caveat: it hints that what is expected may not come about. So in forecasts it sounds more cautious than will.
Negatives on the scale
Negative forms have their own order too, and it is not a mirror of the positive one.
That can't be right.
He may not know.
It shouldn't take long.
Note that mustn't is not used for inference at all. The negative conclusion is can't.
The mistakes everyone makes
The first is taking mustn't for a negative inference.
He mustn't be home. → He can't be home.
The second is reading should as a guarantee.
The parcel should arrive today.
The third is missing the inference inside will.
That will be the courier.
All three are caught by one question. How far am I prepared to stand behind this claim?
Try it right now
Look at a situation around you and describe it in English in three sentences from different points on the scale.
Then work out for yourself what each rests on: evidence, expectation or a guess.