Life is full of mysteries, and sometimes you have to be a detective! Modals of deduction are your tools for making logical guesses and expressing how certain you are about something, whether it’s happening now or happened in the past. 🕵️♀️
Function & Usage
We use these modal verbs to make a guess or conclusion based on evidence. The modal you choose shows your level of certainty.
- Must (Strong Certainty – Positive): You are almost 100% sure something is true.
Example (Present): He’s been yawning all day. He must be tired.
Example (Past): The ground is wet. It must have rained last night. - Can’t (Strong Certainty – Negative): You are almost 100% sure something is impossible.
Example (Present): That can’t be the right answer. It’s too easy.
Example (Past): She can’t have finished the book already! She only started it an hour ago. - Might / Could / May (Possibility / Weak Certainty): You think something is possible, but you are not sure (around 50% certain).
Example (Present): I’m not sure where Sarah is. She might be in a meeting.
Example (Past): The traffic was bad. They might have missed their flight.
The Structure (Form)
The structure changes depending on whether you are making a guess about the present or the past.
| Time | Structure | Example |
|---|---|---|
| Present | Modal + Base Verb | She must be sick. / They might know. |
| Past | Modal + have + Past Participle | She must have been sick. / They might have known. |
Common Mistakes to Avoid
- Using the wrong verb form for past deductions: Always use `have + past participle`. Incorrect: “He must have went home.” Correct: “He must have gone home.”
- Confusing `can’t` and `mustn’t`: For negative deductions, we use `can’t`, not `mustn’t`. `Mustn’t` is for prohibition. Incorrect: “That mustn’t be true.” Correct: “That can’t be true.”
Let’s see it in action! 🎬
Person A: The lights are on in John’s apartment.
Person B: He must be home then. Let’s go say hello.
Friend 1: I can’t find my phone anywhere!
Friend 2: You might have left it in the car. We should go and check.
Person A: Sarah isn’t answering her phone.
Person B: That’s strange. She can’t have forgotten about our meeting, she was so excited about it.
Check Your Understanding! ✅
1. (100% sure) He looks very unhappy. He a bad day. (have)
2. (100% impossible) You just ate a huge lunch! You hungry again. (be)
3. (50% possible) I’m not sure why she is late. She stuck in traffic. (be)
4. (100% sure) The computer is off. Someone it off. (turn)
5. (100% impossible) He studied for weeks. He the exam. (fail)
6. It ___ rained last night. The streets are all wet.
7. “Where is my wallet?” “I’m not sure. You ___ left it at the restaurant.”
8. You haven’t eaten all day. You ___ be hungry.
9. That ___ be Maria’s car. Hers is blue, not red.
10. He ___ the email. He never checks his inbox.
