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B1 Grammar ⏱️ 12 mins

Detective Case File #404

🕵️ Modals of Deduction: Solving the Mystery

Master how to make smart deductions in the present using must, might / could, and can't.

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Teacher's Brief

• Allocate 12 minutes for students to study deduction formulas and complete the 4 mystery stages.

• Project the clue board and let learners check in pairs on the trap between can't be and mustn't be.

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Learner's Mission

• Scan the 4 rule cards below to discover certainty levels (from 95% sure to 0% impossible).

• Crack all 12 case questions using the single-word clue tags in brackets like [certain] or [now].

ancient, dusty castle interior

RULE BOARD

Deduction Decoder: 4 Key Formulas

Target Structure	When to Use	Detective Example
Must + be	For when you are almost sure it's true (~95% certain).	"That strange noise must be the wind."
Might / Could + be	For when it's possible but you're not sure (~50% chance).	"The old map might be a clue."
Can't + be	For when you are almost sure it's not true (impossible).	"It can't be real treasure. The box is empty."
Modal + be + verb-ing	For something happening right now at this exact moment.	"Someone must be hiding in the ancient castle."

COMMON TRAP

Watch Out: "Can't be" vs. "Mustn't be"

To say something is logically impossible, English detectives never use mustn't be. Always use can't be!

✅ CORRECT (Deduction)

"The ghost story can't be true; it is just a local legend."

❌ INCORRECT (Not allowed, not deduction)

"The ghost story mustn't be true..." (Wrong! 'Mustn't' means forbidden!)

detective examining a strange map

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Detective Practice Tasks

12 Questions • 4 Investigative Stages

Stage 01 Choose the Correct Sentence

1. The museum experts tested the crown and confirmed the diamonds are 100% genuine. [certain]

Select the correct deductive statement:

____ a. This ancient treasure might be authentic. ____ b. This ancient treasure must be authentic. ____ c. This ancient treasure can't be authentic.

2. We found strange markings on the stone wall, but nobody knows who made them. [possible]

Select the correct deductive statement:

____ a. The markings could be a clue to solve the mystery. ____ b. The markings must be a clue to solve the mystery. ____ c. The markings can't be a clue to solve the mystery.

3. Look at that photo: it is clearly a white sheet on a stick, not a real ghost! [impossible]

Select the correct deductive statement:

____ a. That blurry figure mustn't be a ghost from the legend. ____ b. That blurry figure might be a ghost from the legend. ____ c. That blurry figure can't be a ghost from the legend.

Stage 02 Rewrite the Sentences

4. "I am sure this missing treasure is ancient." [sure]

Complete the sentence:

This missing treasure ____ .

5. "Perhaps this strange old book is a hidden clue." [maybe]

Complete the sentence:

This strange old book ____ .

6. "I am certain this simple theory isn't the answer to the mystery." [impossible]

Complete the sentence:

This simple theory ____ the answer to the mystery.

Stage 03 Fill in the Gaps (Modal + be / Modal + be + V-ing)

7. All scientific facts support Professor Miller's theory; it is the only logical explanation. [logical]

Fill in the blank with the correct modal verb form (must be / could be / can't be):

His scientific theory ____ correct.

8. Listen! Footsteps are echoing right now along the ancient castle corridor! [now]

Fill in the continuous modal deduction (modal + be + verb-ing with hide):

Someone ____ in the dark corridor right now.

9. Look inside the antique iron chest — there are only spiderwebs and dust, completely empty! [empty]

Fill in the gap to show impossibility:

The hidden pirate treasure ____ in this chest.

Stage 04 Error Correction in Messages

💬 Detective Leo: "I checked the old tower with thermal cameras. That shadow mustn't be a ghost; legends aren't real."

10. Fix the incorrect negative deduction [wrong-negative]:

Type the single corrected phrase to replace "mustn't be":

____

💬 Detective Maya: "I hear keystrokes from the office right now. The code writer must solve the ancient code at this exact moment!"

11. Change the simple modal to the continuous form for right now [now]:

Type the corrected continuous phrase to replace "must solve":

____

💬 Detective Sam: "Don't discard his strange theory so quickly. It can be true, though nobody has proved it yet."

12. Correct "can be" to express uncertain possibility [possibility]:

Type a modal phrase that means 'perhaps':

____

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Case Solved: Remember the certainty levels — must be (95% sure it is), might/could be (50% maybe), can't be (95% sure it is NOT), and modal + be + verb-ing (happening this instant)!

Modals of Deduction: Solving the Mystery

🕵️ Modals of Deduction: Solving the Mystery

Teacher's Brief

Learner's Mission

Deduction Decoder: 4 Key Formulas

Watch Out: "Can't be" vs. "Mustn't be"

Detective Practice Tasks

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