Speaking: Grammar in Speaking B2 - Lesson 5: Using Modals for Speculation (must be, might have)

Speaking: Grammar in Speaking B2 - Lesson 5: Using Modals for Speculation

Welcome to our final lesson! In everyday conversation, we rarely know everything with 100% certainty. We constantly have to make guesses and logical deductions. Using modals of speculation¹ is the key to expressing *how sure* you are about something. Mastering this grammar will make your English sound much more natural and thoughtful.

Speculating about the PRESENT

Use this structure to make a logical guess about a situation happening now, based on evidence.

Structure: Modal + be / Modal + base verb

The Certainty Scale:

~95% Sure (Logical Conclusion) → `must be`

"He's not answering his phone. He must be in a meeting."

~50% Sure (Possibility) → `might be` / `may be` / `could be`

"I don't know where my keys are. They might be in the car, or they could be on my desk."

~95% Sure it's Impossible → `can't be`

"You just had lunch! You can't be hungry again already."

Speculating about the PAST

Use this structure to make a logical guess about a situation that already happened.

Structure: Modal + have + past participle

The Certainty Scale:

~95% Sure → `must have` + p.p.

"The ground is wet. It must have rained last night."

~50% Sure → `might have` / `may have` / `could have` + p.p.

"He was late for work. He might have gotten stuck in traffic."

~95% Sure it was Impossible → `can't have` + p.p.

"She got the highest score on the test. She can't have studied very little."

Scenario: The Office Mystery

Listen to two colleagues trying to figure out why their manager seems upset. Notice how they use modals to show their level of certainty².

Sophea: "The boss seems really quiet and unhappy today. What's wrong?"

Vannak: "I know. He must be stressed about the upcoming deadline. The project is a little behind schedule." (Present speculation - 95% sure)

Sophea: "That's true. Or he could have received some bad news this morning. He didn't say hello when he came in." (Past speculation - 50% sure)

Vannak: "No, he can't have received bad news. I just saw him smiling on the phone a minute ago. It must be about the project." (Past speculation - 95% sure it's impossible; Present speculation - 95% sure)

Common Mistake: `can't` vs. `mustn't`

A very common error is using `mustn't` for negative speculation. We use `mustn't` for strong obligation or prohibition (You mustn't smoke here).

For logical deduction (when you are sure something is impossible), always use `can't`.

• Incorrect: "That story mustn't be true."
• Correct: "That story can't be true."

• Incorrect: "He mustn't have seen me."
• Correct: "He can't have seen me."

Practice Quiz: How Certain Are You?

Read the situation and choose the modal of speculation that fits the context best.

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1. Your friend didn't come to your party, even though she promised she would. The next day, you can't reach her. You think, "Something serious __________."

A) might happen
B) must have happened
C) can't have happened

→ Answer: B. You are making a strong logical deduction³ about a past event based on the evidence.

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2. You find a new, expensive-looking pen on the floor of your classroom. It __________ to the teacher, or maybe another student.

A) must belong
B) can't belong
C) could belong

→ Answer: C. You are expressing a possibility, as you are not sure who the owner is. 'Might belong' or 'may belong' would also be correct.

Your Mission: The Detective Challenge

Your mission is to practice being a detective and speculating about a situation.

1. Find a simple, unexplained situation. For example: Your favourite street food vendor isn't in their usual spot today.
2. Make at least three speculative statements about why, using modals from the lesson to show different levels of certainty.
3. Record yourself for 60 seconds explaining your theories.
   Example: "My usual coffee seller isn't here this morning. That's odd. She must be sick, because she is always so punctual⁴. Or maybe she could have decided to take a holiday. No, she can't have gone on holiday without telling her regular customers. I'm sure she's just sick."
4. This exercise trains you to use this grammar flexibly and spontaneously in real-world situations.

Vocabulary Glossary

1. Modal of Speculation: (Noun Phrase) - កិរិយាស័ព្ទជំនួយសម្រាប់ការសន្និដ្ឋាន - A modal verb⁵ (must, may, might, could, can't) used to make a logical guess or deduction. ↩
2. Certainty: (Noun) - ភាពប្រាកដប្រជា - The state of being completely sure about something. ↩
3. Deduction: (Noun) - ការសន្និដ្ឋាន - A conclusion that you reach based on logical reasoning and evidence. ↩
4. Punctual: (Adjective) - ទៀងពេល - Doing something or arriving at the arranged or correct time; not late. ↩
5. Modal Verb: (Noun Phrase) - កិរិយាស័ព្ទជំនួយ - A helping verb (like must, can, should) that expresses ability, possibility, permission, or obligation. ↩