Grammar: ⚙️ Verbs in Depth: 🎭 Modals & 🌱 Non-Finite Verbs (Advanced) (B2) - Lesson 1: Modals of Deduction & Speculation - Present (must be, might be, can't be)

✨ Lesson 1: Modals of Deduction & Speculation (Present)

Welcome to a new B2 module! In life, we don't always know things with 100% certainty. We often have to look at the evidence and make a logical guess, or a deduction¹. In English, we use a special group of modal verbs for this kind of speculation². They show how certain we are about a situation happening now.

The Scale of Certainty

Think of deduction on a scale from 100% sure to 0% sure. The modal verb you choose shows your level of certainty³.

~95% Sure (Strong Deduction): use must be

Use 'must be' when you have strong evidence and you are making a logical⁴ conclusion⁵.

"The lights are on in Sothea's house. She must be home."

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~50% Sure (Possibility): use might be / may be / could be

Use these when something is possible, but you are not sure. There are other possibilities.

"I'm not sure where my keys are. They might be in the car, or they could be on the kitchen table."

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~5% Sure (Strong Negative Deduction): use can't be

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!"

Structure with Action Verbs

This structure also works with action verbs, not just the verb 'be'.

State: "He is very pale. He must be sick."

Action: "He owns three motorbikes. He must like riding very much."

Continuous Action: "Don't go in his room. He must be studying for his exam."

Important: 'mustn't' vs. 'can't'
Do not confuse 'mustn't' and 'can't' for deductions. 'Mustn't' is for prohibition (a rule against something). 'Can't' is for logical impossibility.
Prohibition: "You mustn't smoke in this restaurant."
Deduction: "He can't be at work. It's Sunday!"

🧠 Practice Quiz: How Certain Are You?

Choose the best modal of deduction for each situation.

1. 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 at home, but not 100% certain.)
2. She has a huge library in her house and reads every day. She _______ love books. (must / can't)
   Answer: must (The evidence is very strong.)
3. 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.)
4. 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.)
5. His moto isn't here. He _______ have gone to the market. (can't / must)
   Answer: must (This is the most logical conclusion based on the evidence.)

📝 Homework: Be a Detective

Look at the situations and write a sentence of deduction using 'must be', 'might be/could be', or 'can't be'.

1. You see your friend outside a cinema.
   (Example: He might be going to see a movie.)
   _________________________________________
2. The restaurant is completely dark and the door is locked.
   (Example: It must be closed.)
   _________________________________________
3. Someone tells you they can fly.
   (Example: That can't be true!)
   _________________________________________

Vocabulary Glossary

1. Deduction: (Noun) - ការសន្និដ្ឋាន (kaa sân'nĭ'tʰaan) - The process of reaching a decision or answer by thinking about the known facts. ↩
2. Speculation: (Noun) - ការស្មាន (kaa s'maan) - The activity of guessing possible answers to a question without having enough information to be certain. ↩
3. Certainty: (Noun) - ភាពប្រាកដប្រជា (phéap praa'kât'prâ'chea) - The state of being completely sure about something. ↩
4. Logical: (Adjective) - មានហេតុផល (méan haet'phâl) - Seeming natural, reasonable, or sensible. ↩
5. Conclusion: (Noun) - សេចក្តីសន្និដ្ឋាន (sach'kdəy'sân'nĭ'tʰaan) - A judgement or decision reached by reasoning. ↩