Grammar: Modal Verbs
B2 Lesson 1: Modals of Deduction (Present)
Why It Matters: Expressing Certainty
In English, we use special modal verbs to show how certain we are. This is a key skill for expressing nuanced opinions and making logical guesses (deductions).
Maybe she is home.
The lights are on. She must be home.
The Scale of Certainty 📖
Think of deduction on a scale. The modal verb you choose shows your level of certainty.
Strong Deduction: ~95% Sure
Use must be when you have strong evidence and are making a logical conclusion.
"The lights are on in Sothea's house. She must be home."Possibility: ~50% Sure
Use might be / may be / could be when something is possible, but you are not sure.
"I'm not sure where my keys are. They might be in the car, or they could be on the kitchen table."Strong Negative Deduction: ~5% Sure
Use can't be when you are almost certain something is impossible or not true.
"That man can't be a doctor. He looks about 16 years old!"💡 Pro Tips
1. Using Action Verbs: This structure also works with action verbs, not just 'be'.
- He owns three motorbikes. He must like riding.
- Don't go in his room. He must be studying. (Continuous)
- Prohibition: "You mustn't smoke here."
- Deduction: "He can't be at work. It's Sunday!"
Practice Your Deduction Skills 🎯
Quiz: How Certain Are You?
Choose the best modal of deduction for each situation.
- The phone is ringing but he isn't answering. He _______ be at home. (must / might not)
→ Answer: might not (It's possible he is not home, but not 100% certain.) - She has a huge library and reads every day. She _______ love books. (must / can't)
→ Answer: must (The evidence is very strong.) - You say you saw a ghost? That _______ be true; ghosts aren't real! (can't / might not)
→ Answer: can't (Expresses strong belief that it's impossible.) - A: "Where's the cat?" B: "I'm not sure. It _______ be sleeping under the bed." (must / could)
→ Answer: could (This expresses a simple possibility.)
Your Grammar Mission ⭐
Be a Detective
Look at the situations and write a sentence of deduction using 'must be', 'might be/could be', or 'can't be'.
- You see your friend outside a cinema.
(Example: He might be going to see a movie.) - The restaurant is completely dark and the door is locked.
(Example: It must be closed.) - Someone tells you they can fly.
(Example: That can't be true!)
Key Vocabulary
- Deduction The process of reaching a conclusion by thinking about the known facts.
- Speculation The activity of guessing possible answers without having enough information to be certain.
- Certainty The state of being completely sure about something.
- Logical Conclusion A judgement or decision reached by reasoning.