Grammar · Lesson 14

Modals of deduction: match the claim to the evidence

A deduction is a conclusion you build from evidence, not a fact you directly know. Use must when the available evidence strongly supports one positive explanation, can’t when reliable evidence strongly rejects an explanation, and might or could while more than one explanation remains possible.

60–70 min coreobservation → inferencemust · can’t · might · could15–20 min listening and retrieval

ObserveSeparate what is known from what is inferred.

CalibrateKeep one or several explanations alive.

BuildChoose state, action, or current activity form.

ReasonPresent a careful fictional evidence briefing.

Can-do: By the end, you can separate an observation from an inference; use must for a strong positive present conclusion, can’t for a strong negative present conclusion, and might / could for possibilities; keep might not weaker than can’t; distinguish deduction must from obligation, deduction can’t from inability, and possibility could from ability; build modal + base form and modal + be + -ing; ask about a possible explanation naturally; and state the evidence and uncertainty behind a responsible conclusion.

Two-minute retrieval

Reactivate three nearby systems

1A posted safety rule creates an obligation: Visitors wear a badge.

2Rina has the skill to operate the sound system: Rina operate the sound system.

3The tutor can see the action happening on a shared practice screen now: Rina the microphones now.

North Hall, 2:10 p.m.

Goal: Coordinator Eli needs to find workshop leader Rina before a 2:15 sound check.

Recorded observations: Reception marked Rina present at 2:07. Her labeled equipment case is beside Studio A. Two staff members heard someone testing a microphone inside Studio A at 2:09. Studio B was checked at 2:10 and is empty and locked.

Rina must be somewhere near Studio A.

She might be inside the studio, or she could be speaking from the hallway.

She can’t be in Studio B.

Which statements report observations, and which report conclusions?
Why does the speaker keep two locations possible after might and could?
Which checked fact strongly rejects Studio B?

Discovery check: fact, inference, and strength

1Reception marked Rina present at 2:07 is presented as .

2Rina must be somewhere near Studio A is .

3She might be inside, or she could be in the hallway keeps .

4She can’t be in Studio B is .

5The decisive evidence for that negative inference is: .

Get to the root: modals show the speaker’s reasoning stance

Must does not turn an inference into a verified fact. It tells the listener that the speaker sees one positive explanation as strongly supported. Can’t tells the listener that an explanation conflicts strongly with reliable evidence. Might and could keep an explanation possible without committing to it.

Do not attach fixed percentages to the modals. Evidence quality, the number of plausible alternatives, and the speaker’s judgment all matter. Two careful speakers can see the same incomplete clues and choose different strengths. A good reasoner states the evidence and is ready to revise the conclusion.

reasoning stance	core form	contextual example
strong positive inference	must + base form	The test sound is coming from Studio A. Someone must be using the microphone.
live possibility	might / could + base form	Rina might be inside. She could be in the hallway.
possible negative	might not + base form	She might not have her phone with her.
strong negative inference	can’t + base form	She can’t be in the checked empty room.

May is also a standard possibility modal: Rina may be inside. It is not a core target in this lesson, but it is not wrong in an appropriate context. This page focuses on present conclusions. Past conclusions such as must have left and might have forgotten belong to the later past-speculation system.

Form check: one modal, then the base form

1Strong present state from several clues: Rina nearby.

2One possible explanation for Eli’s quick route choice: He a shorter route.

3One possible possession; the bag has not been checked: Rina the spare key.

4Strongly rejected present location after Studio B was checked empty: She in Studio B.

5Possible activity in progress; voices are audible but not identified: She the microphones.

6Strongly rejected activity now; the locked shop is visibly empty: The owner a customer inside.

Calibrate the conclusion, not a memorized percentage

Use must when independent clues converge and no serious alternative is currently visible. Use might or could when at least one plausible alternative remains. Use can’t when an explanation conflicts with reliable information. A missing clue alone is rarely a contradiction.

Strength check: what does the evidence actually support?

1Rina checked in three minutes ago; her labeled case is by Studio A; test audio came from that area one minute ago. The strongest careful conclusion is that she .

2No one can see whether the voice is inside or just outside the studio. One open possibility is that she .

3The same incomplete evidence supports another live possibility: She .

4Rina has not answered one call, but nobody has checked whether her phone is with her. She .

5Studio B was checked at 2:10 and is empty and locked. Rina .

6A different staff member’s coat is on a chair, with no name or current schedule. The coat alone provides .

Separate deduction from obligation, ability, and general possibility

The modal does not carry one meaning by itself. Visitors must sign in is a rule; Rina must be here is an inference. Rina can use the mixer describes ability; Rina can’t be in Studio B rejects a present location. Affirmative can can describe general possibility, as in The hall can get noisy, but for one uncertain explanation now, use might or could.

For a clear strong negative deduction in this lesson, use can’t. Mustn’t commonly expresses prohibition in U.S. English: Visitors mustn’t enter. Full must not can sometimes express a negative inference, but it can be ambiguous; context and intonation matter. This lesson does not label every inferential must not impossible.

Function check: the surrounding situation decides

1Several fresh clues point to Rina’s location: She must be near Studio A expresses .

2A posted rule says All visitors must sign in. Here must expresses .

3Eli has watched Rina operate the system successfully: Rina can use the mixer describes .

4The hall often becomes noisy during large events: The hall can get noisy describes .

5A safety sign says Visitors mustn’t enter the equipment room. In this context, mustn’t expresses .

Make evidence strong enough for the claim

First report the observation precisely. Then ask whether it supports one explanation, several explanations, or a contradiction. Do not treat absence of evidence as evidence of absence: one unanswered call does not prove that a person can’t be nearby. Do not use stereotypes, appearance, accent, nationality, age, gender, disability, or other identity assumptions as evidence.

In work, health, safety, legal, and relationship contexts, label uncertainty and verify before acting. The message might be delayed is safer than presenting an unsupported technical claim as fact. Grammar cannot make weak evidence responsible.

Evidence check: reason only from what the case supplies

1A staff member opens Studio B, checks it, then locks it again. The direct observation is: .

2From that current checked fact, the strong negative inference is: .

3A brief sound comes from Studio A, but its source is not visible. The calibrated conclusion is: .

4An unlabeled bag sits near the studio, and nobody knows who owns it or when it arrived. .

5A speaker guesses someone’s job from that person’s accent. The responsible response is: .

Choose a present state, repeated fact, or activity in progress

Use modal + be before a noun, adjective, or location: She might be the trainer. She must be busy. She could be upstairs. Use modal + base verb for knowledge, possession, or a general action: She might know the code. She could have the key. She must work nearby. Use modal + be + -ing for an activity viewed in progress now: She must be testing the microphone.

Viewpoint check: what kind of present meaning is inferred?

1Possible current location: The elevator record shows several staff went up, but not who. Rina .

2Possible present knowledge: Rina helped install the keypad, so she .

3Inference about a regular job, not this exact moment: For six months, Rina has entered North Hall with a staff badge every weekday, used an employee desk, and stayed for the full workday. She .

4Inference about an activity happening at 2:10: Her recent check-in, labeled case, and a live test signal carrying her microphone label all converge. She .

5One possible activity during the noon break; no one has checked: The team .

Ask about possibilities without turning them into requirements

Could Rina be in the hallway? and Might Rina be inside? ask whether an explanation is possible, although might questions can sound more formal. In everyday conversation, Do you think Rina might be inside? is natural. Must Rina be inside? often sounds like a question about necessity, so it is not the clearest everyday way to ask how likely the deduction is.

Question check: ask about an explanation

1Ask directly whether the hallway is one possible location:

2Use a natural conversational frame with might:

3Without supporting context, Must Rina be inside? can invite .

In neutral speech, the modal is often lighter than the conclusion: She must be near STUdio A, She might be in the HALLway. Stress moves when you correct the strength: She MIGHT be inside, but she doesn’t HAVE to be. There is no rule that the same word always receives stress.

In connected U.S. speech, the final consonant of a modal may link into be: must_be, might_be, could_be. The /t/ in must, might, and can’t may be light or unreleased, but the negative meaning of can’t must remain recoverable from the vowel, context, and stress. Do not add to: must to be.

Listen for evidence, stance, and connected form

1. What is the discourse job of the first line?

2. What stance does must be near Studio A express?

3. What remains open in the third line?

4. Why does the speaker use can’t in the fourth line?

5. What does contrastive stress do in the final line?

Read a complete evidence update

Eli’s 2:11 p.m. fictional update

Reception marked Rina present at 2:07, and her labeled equipment case is beside Studio A. Two staff members heard a microphone test from that area at 2:09, so she must be nearby. She might be testing the wireless microphone inside, or she could be checking its signal from the hallway. She can’t be in Studio B because a staff member checked that room at 2:10 and found it empty and locked. Rina has not answered one call, but that does not prove she is unavailable. She might not have her phone with her. Eli is walking to Studio A to verify the conclusion.

Reading check: use only the written evidence

1. Why does Eli say Rina must be nearby?

2. Which two explanations remain possible?

3. What evidence rules out Studio B?

4. Why does Eli avoid saying Rina can’t be available?

Build one open possibility and one rejected explanation

Build 1: preserve uncertainty

Offer one unconfirmed explanation for the sound.

Build 2: state the contradiction

Reject Studio B and include the evidence.

Notice and repair the reasoning form

Repair 1: remove the infinitive marker

Tap the word that does not belong after the modal.

Repair 2: distinguish deduction from prohibition

Studio B was checked empty and locked. Tap the modal that gives the wrong core meaning for this controlled conclusion.

Repair 3: build the current activity

Tap the finite form that must return to the base form after might.

Play and practice

Games and a short reading that recycle this lesson. Use them as a warm-up, a fast finisher, or homework. Each one checks itself.

Read

Where is the new printer?

Read once for meaning. Then read again and do the tasks below.

Office story167 wordsAbout 1 min

It is 11:00, and Priya is looking for the office’s new printer. The delivery app shows that it arrived at 9:40, but it is not at the front desk. Office rules say that all deliveries must go to the front desk first.

“It must be somewhere in the building,” Priya says. “The driver took a photo of the box inside our main door.” Her coworker Sam looks at the photo. “It could be in the mailroom, or someone might have it in the storage area.”

Priya calls the mailroom, but nobody answers. “They might be at lunch,” Sam says. “It can’t be in the storage area, though. That room is locked, only the manager has the key, and she is on vacation this week.”

Then Priya notices a large box under the coat rack near the main door. It has the company logo on it. “That must be our printer!” she says. She opens the box and finds the printer and a short note from the driver.

Find it

Tap the 6 modals that show a deduction or a possibility. Skip must when it states a rule.

Check your understanding

1. Why does Priya think the printer is in the building?

2. Why can’t the printer be in the storage area?

3. Where is the printer in the end?

Sort it

Deduction, rule, or ability?

What does the modal do in each sentence? Tap a sentence, then tap its box.

Unscramble

Build careful conclusions

Tap the words in order to build the sentence. Tap a placed word to send it back.

1. Strong conclusion: the lights are off, and the door is locked.
2. Reject a place you have already checked.
3. Give one possible reason he is late.
4. Ask about a possibility in a natural way.

Match up

Evidence and conclusion

Tap an item on the left, then its partner on the right.

Evidence

Conclusion

Use it without investigating a real person

You never need to infer anything about a real coworker, client, family member, neighbor, health condition, disability, relationship, location, schedule, finances, legal matter, identity, accent, nationality, password, message, or private behavior. Use Rina, Eli, North Hall, and the fictional evidence cards on this page. Real details are optional.

Observation cards: Create five neutral fictional clues about rooms, equipment, schedules, sounds, or written records. Label each observation before drawing a conclusion.

Possibilities: Keep two explanations alive with might and could. Say what additional evidence would distinguish them.

Strong conclusions: Use must only when several clues converge. Use can’t only when a current reliable fact directly contradicts an explanation.

Responsible close: State one uncertainty, ask one natural possibility question, and explain how you would verify rather than present an inference as fact.

Final production and check

Final production: give a 75–90 second fictional evidence briefing

Present at least four observations about a neutral workshop mystery. Separate the observations from your conclusions. Give two live possibilities with might and could, one possible negative with might not, one strong positive inference with must, and one strong negative inference with can’t. Include one state and one activity in progress. End with a natural question and the evidence you would verify next.

Evidence check: Can the listener name the exact clue behind every conclusion, identify which alternatives remain live, distinguish a contradiction from missing information, recover every modal plus base-form chain, and tell which statement is observation rather than inference?

Final check: defend the reasoning strength

1. Reception checked Rina in at 2:07, her case is beside Studio A, and staff heard a test there at 2:09. Which conclusion matches the converging evidence?

2. Rina has not answered one call, and nobody has checked her phone. Which conclusion is appropriately tentative?

3. A staff member checked Studio B at 2:10 and found it empty and locked. Which conclusion states the contradiction?

Reflect: how strong is the evidence, not just the modal?

Explain the decision path

Complete these ideas aloud: An observation is... An inference is... I use must when... I use can’t when... I keep might or could when... Might not differs from can’t because...

Take one fictional clue and produce three responses: an unsupported overclaim, a calibrated possibility, and a strong conclusion after adding decisive evidence. Explain what changed in the evidence, not only in the modal.

Return tomorrow for the three-item retrieval check before rereading the lesson.

Next-day retrieval

Return without rereading the page. Name the observation, list any remaining alternatives, and identify a direct contradiction before selecting a modal.

Three delayed checks

1. You hear one brief sound from a closed room, but you cannot identify its source. Which conclusion preserves uncertainty?

2. Which sentence accurately presents one possible current activity?

3. Compare Her fresh test file is open, so she must be working nearby with Visitors must sign in. What changes?