Speculation and Deduction: Modal verbs
Express certainty, possibility, and make deductions in English.
- check_circleI can express high certainty using must and can't.
- check_circleI can express possibility using might, may, and could.
- check_circleI can make deductions about the past using modal + have + past participle.
Discover
Meet today's English in a real situation — no rules yet. Read it once and try to guess the pattern in the words in bold. Underlined words open a short definition.
Emma: Have you seen Liam today? He wasn’t in the morning meeting.
Noah: I’m not sure. He might be working from home. I know he wasn’t feeling well yesterday.
Emma: He can’t be working from home. His laptop is still on his desk. Look!
Noah: Oh, you’re right. That’s strange. Well, he must have stepped out for a coffee or something.
Emma: Maybe, but his jacket is still on his chair. He couldn’t have gone far without it; it’s freezing outside.
Noah: True. What if he had an emergency? He might have left in a hurry.
Emma: I suppose so. Or he could be in a meeting with the boss. They sometimes have sudden, urgent discussions.
Noah: He is bound to come back soon if all his things are here. I’ll ask the receptionist, she must have seen him leave.
Emma: Good idea. It’s quite a mystery!
Learn
Now the rules behind what you just saw — explained simply, with examples. Underlined words open a short definition — hover on desktop, tap on a phone.
Speculating and making deductions is about expressing how sure you are about something based on evidence. In English, we use different modal verbs depending on our level of certainty and whether we are talking about the present or the past.
Present Deduction
When we have evidence about a present situation, we use these modals:
- Must: You are very sure something is true.
- Might / May / Could: You think something is possible, but you aren’t sure.
- Can’t: You are very sure something is not true.
A: Whose coat is this? B: It must be John’s. He’s the only one who wears that size. A: It could be David’s. B: It can’t be Sarah’s. It’s too big.
Notice that the opposite of must (for deduction) is can’t, not “mustn’t”. “Mustn’t” means prohibition.
Past Deduction
To speculate or make deductions about the past, we use: Modal + have + past participle.
- Must have + past participle: You are very sure something happened.
- Might / May / Could have + past participle: You think it’s possible something happened.
- Can’t / Couldn’t have + past participle: You are very sure something did not happen.
They are very late. They might have missed the train. She got top marks in the exam. She must have studied hard. He can’t have been at the party. I saw him in a different city!
A common mistake is using “to” after the modal. Remember that modals are followed directly by the bare infinitive.
- Incorrect: He must to have left.
- Correct: He must have left.
Other Expressions for Speculation
Besides modals, we can use other expressions to show certainty or uncertainty:
- Be bound to: Very likely or certain to happen. (e.g., It’s bound to rain later.)
- Be likely to / Be unlikely to: Probable / not probable. (e.g., She is likely to win the award.)
- Present 100% Yes: must + base verb
- Present 50%: might / may / could + base verb
- Present 100% No: can’t + base verb
- Past 100% Yes: must have + past participle
- Past 50%: might / may / could have + past participle
- Past 100% No: can’t / couldn’t have + past participle
Practice
Try it yourself. You'll see right away whether you got it right, plus a short explanation of why.
Use It
Now make the language yours in a real task. Use the prompt below — the editor keeps a simple word count, and nothing is saved or graded.
Before you finish — be honest. Can you do these now?