Use modals of deduction to guess what's true based on the evidence in front of you. Must means you're almost sure it's true, can't means you're almost sure it isn't, and might, may and could mean it's possible.
The pattern
She must be at the gym.
He might have missed the bus.
They can't have seen the email.
01Guessing about now
Use modal + base verb for states, and modal + be + -ing for something in progress right now: He must be working late.
| How sure? | Modal | Example |
|---|---|---|
| Almost sure it's true | must | The lights are on. They must be home. |
| Possible | might / may / could | She might be stuck in traffic. |
| Possibly not | might not / may not | He may not know about the change. |
| Almost sure it's false | can't / couldn't | That can't be Jake. He's in Denver this week. |
02Guessing about the past
For the past, use modal + have + past participle. The modal shows how sure you are now; have + participle moves the guess into the past.
- The streets are wet. It must have rained last night.
- She didn't pick up. She might have been in a meeting.
- He can't have finished already. He only started ten minutes ago.
- They couldn't have known about the surprise party.
03Watch out for these
Could and couldn't are not opposites here. Could means maybe; couldn't means impossible.
In American English, must not is common for a negative guess: "He must not like spicy food. He didn't touch it." The contraction mustn't is rare in the US. For something you think is impossible, can't is the strongest choice.
Don't use had to for guesses about the past. Had to is about obligation; use must have.
04Common mistakes
She must to be tired.
She must be tired.
Modals are followed by the base verb, without to.
They must went home.
They must have gone home.
For past guesses, use modal + have + past participle.
He had to forget my birthday.
He must have forgotten my birthday.
Had to means obligation; must have is for a confident guess about the past.
She must be work late.
She must be working late.
For something in progress, use must be + -ing.