English — Modals of present deduction
To draw logical conclusions about a present situation, English uses modal verbs. The choice shows how sure the speaker is: must for certainty, can't for impossibility, and may / might / could for a mere possibility, as in He must be tired.
How it works
The modal + the base form (or be + -ing) states a deduction, not a fact. Use must when the evidence makes the conclusion certain (She must know him); can't (or couldn't) when it is logically impossible (That can't be true); and may, might or could when it is only possible (He might be at work). Note that the opposite of must here is can't, not mustn't.
Certainty scale
| Modal | Meaning | Example |
|---|---|---|
| must | certain — the evidence points to one conclusion | He must be tired. |
| must be + -ing | certain, action in progress now | She must be joking. |
| may / might / could | possible — one of several options | He might be at work. |
| may not / might not | possibly not — one negative option | She may not know. |
| can't / couldn't | impossible — the conclusion is ruled out | That can't be true. |
| can't be + -ing | impossible, action in progress now | They can't be sleeping. |
Good to know
Do not confuse deduction with obligation. Mustn't means "it is forbidden" (You mustn't smoke here), never "it is impossible" — for that use can't (It can't be him). Likewise, deductive must has no past or future of its own: for the past use must have + past participle (He must have left).