Modals of Deduction: Must, Might, Could, Can't
Learn how to make logical guesses about present situations using must, might, could, and can't.
- check_circleI can express strong certainty about a present situation.
- check_circleI can express impossibility about a present situation.
- check_circleI can express possibility when I am not 100% sure.
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.
A conversation between two flatmates who hear a strange noise in the middle of the night.
Leo: Did you hear that noise? It sounded like someone downstairs.
Sam: It must be the cat. You know how she knocks things over.
Leo: No, it can’t be the cat. She is sleeping right here on my bed.
Sam: Really? Then it might be a burglar! Should we call the police?
Leo: Wait, don’t panic. It could be our other flatmate, Tom. He sometimes works late.
Sam: It can’t be Tom. He texted me earlier that he was staying at his girlfriend’s house tonight.
Leo: What a mystery! Oh wait, I left the TV on. It must be the movie I was watching.
Sam: Phew! You scared me.
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.
When we want to make a logical guess or deduction about a situation based on evidence, we use modal verbs of deduction. The verb we choose depends on how certain we are.
100% Sure it is True: Must
When you are almost completely sure that something is true based on the facts you have, use must + base verb.
- “You’ve been travelling all day. You must be exhausted!”
- “He has three expensive cars. He must make a lot of money.”
Do not use must for obligations here. In deduction, “You must be tired” means “I logically conclude you are tired,” not “I order you to be tired.”
100% Sure it is False: Can’t
When you are almost completely sure that something is impossible or not true, use can’t + base verb (or cannot).
We use can’t as the opposite of must for deductions. Do not use mustn’t.
- “That can’t be John. John is in London this week!”
- “She can’t be hungry. She just ate a huge pizza.”
50% Sure: Might / May / Could
When you think something is possible but you aren’t completely sure, use might, may, or could. These three words have almost the exact same meaning when making deductions.
- “Where is my wallet? I don’t know, it might be in the car.”
- “That woman could be his sister. They look very similar.”
- “Take an umbrella. It may rain later.”
Summary Pattern
| Certainty | Modal | Example |
|---|---|---|
| 99% True | Must | He must be sick. |
| 50% Possible | Might / May / Could | He might be sick. |
| 99% False | Can’t | He can’t be sick. |
Notice that after all these modals, we use the base form of the verb without “to” (e.g., must be, not must to be).
A common mistake is translating directly from Spanish “debe ser” vs “no debe ser”. Spanish: “Ese no debe ser su auto.” English: “That can’t be his car.” (Not mustn’t be).
- Use must when you are sure something is true.
- Use can’t when you are sure something is impossible. (NOT mustn’t)
- Use might, may, or could when you think something is possible but aren’t sure.
- Always use the base verb after the modal without “to” (must be, can’t have).
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?