🕵️ Lesson 4.2: Modals of Deduction & Obligation
Great speakers don't just describe what they know — they reason out loud about what they don't know. In this lesson you'll become a detective, using modal verbs to say how sure you are: "He must be tired," "That can't be right," "She might be at work." Then we'll tie together the obligation modals you met at A2, so your whole modal toolkit sits in one place.
📚 What You'll Learn
By the end of this lesson, you will be able to:
- Use must and can't to make logical deductions (near-certainty) about the present
- Use might, may and could to talk about possibility when you're not sure
- Place any situation on a certainty scale, from "sure yes" to "sure no"
- Review and consolidate the obligation modals (have to, must, don't have to, mustn't, should)
⏱️ Estimated Time: ~55 minutes
🎯 Project: Play detective — look at a set of situations and clues and make at least six spoken deductions ("He must be tired," "It can't be true," "They might be lost").
In This Lesson
Thinking Out Loud
Imagine you see a friend's coat and umbrella by the door, wet from the rain, and her coffee cup is still warm. You don't know she's home — nobody told you — but the clues point one way, so you say: "She must be home." That's deduction: using evidence to reach a confident conclusion.
At A2 you used modals to talk about rules and advice (you must wear a seatbelt, you should rest). At B1 the very same little words — must, can't, might, may, could — get a powerful new job: measuring how certain you are. This is one of the most natural, everyday things English speakers do, and once you can do it, your conversation sounds far more thoughtful and alive.
🧠 Mindset
Deduction is guessing with reasons, and that means you can't be "wrong" for having an opinion — you can only be more or less sure. So this is a wonderfully low-pressure grammar point to speak with: even a shaky guess ("It might be…") is perfectly correct English. Give yourself permission to think out loud in your new language. The learners who talk through their reasoning, mistakes and all, are exactly the ones who become fluent. 🌱
Deduction: must, can't, might
Deduction about the present uses a very simple form:
📖 The form
subject + modal + base verb
She must be tired. · It can't be true. · They might know.
The modal never changes for he/she/it (no -s), and the verb after it is always the plain base form — very often be. So: "He must be hungry," never "He must is hungry."
Each modal expresses a different level of certainty. Here's what each one really means:
| Modal | What you mean | Example (with the clue) |
|---|---|---|
| must | I'm almost certain it IS true — a positive deduction from evidence. | The light's on — she must be home. |
| can't | I'm almost certain it is NOT true — a negative deduction. | He can't be hungry — he's just eaten. |
| might / may / could | It's possible, but I'm not sure either way. | She's not answering — she might be busy. / It could be a mistake. |
Must and can't are the two ends — the confident conclusions. Might, may and could sit in the middle and are almost interchangeable: all three simply mean "maybe." (In everyday speech might and could are the most common; may feels a touch more formal.)
⚠️ The number-one deduction mistake
The opposite of must (for deduction) is can't — not mustn't.
To say you're sure something is false, use can't:
✅ "He can't be at home — his car's gone."
❌ "He mustn't be at home."
Mustn't exists, but it means "it's forbidden" (obligation), not "it's impossible" (deduction). Keep them apart — we'll compare them directly below.
The Certainty Scale
Picture your certainty as a line running from a confident yes down to a confident no. The modal you pick shows exactly where you stand.
| How sure? | Modal | Example |
|---|---|---|
| ✅ Sure it's YES (~95%) | must | He must be rich — look at that car. |
| 🤔 Maybe (~50%) | might / may / could | He might be rich, or he could just be borrowing it. |
| ❌ Sure it's NO (~95%) | can't | He can't be rich — he never has any money. |
graph LR
A["MUST
sure YES"] --> B["MIGHT / MAY / COULD
maybe"]
B --> C["CAN'T
sure NO"]
✅ Quick self-test
Before you speak, ask: "How sure am I?" Sure it's true → must. Sure it's false → can't. Genuinely unsure → might / may / could. If you can point to the clue that makes you sure, you've almost certainly picked the right modal.
One Word, Two Jobs
Here's the idea that ties the whole lesson together. The word must does two completely different jobs, and context tells you which one is meant.
| Job | Meaning | Example |
|---|---|---|
| Obligation (A2) | It's necessary / it's a rule. | You must wear a seatbelt. |
| Deduction (new at B1) | I'm almost sure this is true. | You must be tired after that trip. |
💡 How to tell them apart
Deductions are guesses about a situation — you're reading clues ("You must be tired" = I can see you look exhausted). Obligation is about rules and necessity — someone requires the action ("You must sign here"). The verb after must is a clue too: with a state verb like be, know, have it's usually a deduction; with an action someone performs, it's usually obligation. In real conversation the situation makes it obvious, so don't worry — you already understood both examples above without thinking.
🔭 A quick peek ahead: To make a deduction about the past, English uses must have + past participle — "She's not here; she must have left." You don't need to produce this yet; it's a B2 skill and we'll teach it properly later. For now, just notice it exists, and keep your own deductions in the present.
Obligation Review
You met these modals back at A2. Now that you're using modals for deduction, it's the perfect moment to consolidate the obligation family so the two uses don't blur together. Here's the compact review:
| Modal | Meaning | Example |
|---|---|---|
| must / have to | necessary — you have no choice | I have to work tomorrow. · You must show ID. |
| don't have to | NOT necessary — but you can if you want | It's Sunday, so you don't have to get up early. |
| mustn't | prohibited — it's against the rules | You mustn't use your phone here. |
| should / shouldn't | advice — a good / bad idea | You should rest. · You shouldn't worry. |
⚠️ The classic trap: mustn't vs don't have to
These feel like opposites of "must," but they mean very different things:
- mustn't = it's forbidden. "You mustn't smoke here" (smoking is not allowed).
- don't have to = it's optional. "You don't have to come" (you're free to stay home).
So "You don't have to shout" (it isn't necessary) is very different from "You mustn't shout" (it's not allowed).
Notice the overlap with deduction: must appears in both tables (obligation and deduction), while mustn't belongs only to obligation. That's exactly why we use can't, not mustn't, for a negative deduction.
Detectives at Work
Listen for how these two friends reason from clues, moving up and down the certainty scale as they find a lost bag's owner:
A: Someone's left a bag on the train. Whose do you think it is?
B: Well, there's a laptop and some business cards inside, so it can't be a child's.
A: True. And there's a train ticket to Manchester — the owner might be going there for work.
B: Look, the coffee's still warm. They must be somewhere on the train right now!
A: Good thinking. They could be in the next car. Let's ask the conductor.
And here's a second everyday deduction — two colleagues wondering why a friend is late:
A: It's nine-thirty and Sam's still not here. That's not like him.
B: He can't be stuck in traffic — he cycles. He might still be asleep.
A: Or he may be unwell. He looked pale yesterday.
B: His coat isn't on his chair, so he must still be on his way. Let's give him ten minutes.
Detective's opener: "There's a clue here — [the clue] — so it must / can't / might be [your conclusion]."
Always link the modal to the evidence. That "so" is the sound of deduction.
🔊 Hear It & Repeat — Model Sentences
Listen to each model sentence, then say it back. Use the 🎧 Audio control (bottom-right) to choose a UK or US voice and slow the speed down. Notice how must, can't, and might show how sure we are, while must and have to show rules.
- She must be tired after such a long journey.
- He can't be at home because his car isn't here.
- They might be stuck in traffic right now.
- You must wear a seatbelt in the car.
- You don't have to pay for the tickets.
Practice & Project
🏋️ Exercise 1: must, can't or might?
Objective: Read the clue and choose the modal that matches your certainty. (Sure it's true → must; sure it's false → can't; unsure → might.)
- Her office light is off and her car's gone. She be at work.
- The ground is wet and people have umbrellas. It be raining.
- I'm not sure who's knocking — it be the mail carrier.
- He's wearing a thick coat and gloves. It be cold outside.
- They've only known each other a day, so they be old friends.
✅ Answer
1. can't · 2. must · 3. might (or could/may) · 4. must · 5. can't
🏋️ Exercise 2: fix the deduction error
Objective: Each sentence uses mustn't where a deduction is meant. Rewrite it correctly with can't (or fix the modal).
- That mustn't be her brother — she doesn't have one.
- The store mustn't be open; all the lights are off.
- He mustn't be Italian — he doesn't speak a word of it.
- This mustn't be the right address; there's no number 40.
✅ Answer
1. That can't be her brother — she doesn't have one.
2. The store can't be open; all the lights are off.
3. He can't be Italian — he doesn't speak a word of it.
4. This can't be the right address; there's no number 40.
Why: for a negative deduction ("I'm sure it's not true"), the opposite of "must" is "can't," never "mustn't."
🏋️ Exercise 3: obligation review gap-fill
Objective: Complete each gap with have to, don't have to, mustn't or should.
- The museum is free, so you pay to get in.
- You touch the paintings — it's strictly forbidden.
- You look exhausted. You get an early night.
- I renew my passport before we travel; it's expired.
✅ Answer
1. don't have to (it isn't necessary) · 2. mustn't (prohibited) · 3. should (advice) · 4. have to (necessary)
🎯 Your Project: Play Detective
Speak out loud — to a partner, in class, or into your phone's recorder. Your goal: make at least six deductions from clues.
- (2 min) Gather your "cases." Use any of these, or invent your own: a stranger's shopping basket; a photo of a messy desk; sounds coming from another room; why a friend hasn't replied to your message; whose lost phone this is.
- (4 min) For each case, state the clue, then your deduction: "There are three dog toys in the basket, so this person must have a dog."
- (3 min) Use all three certainty levels at least once: a must (sure yes), a can't (sure no), and a might / may / could (maybe).
- (1 min) Add one obligation sentence to show the contrast: "If it's a lost phone, I should hand it in — I mustn't just keep it."
- (1 min) If you recorded it, listen back: did you ever say mustn't when you meant can't?
✅ Project Completion Checklist
- ☐ I made at least six deductions
- ☐ Each deduction was linked to a clue ("…, so it must be…")
- ☐ I used must, can't, and might/may/could at least once each
- ☐ I used can't (not mustn't) for a negative deduction
- ☐ I added one obligation sentence for contrast
🎯 Quick Quiz
Question 1: "The whole house is dark and silent. They ___ be asleep." Which word best completes the deduction?
Question 2: The sign says entry is free. Which sentence is correct?
Tips & Common Mistakes
✅ Do's
- Always tie your deduction to a clue — the word "so" ("…, so she must be…") makes your reasoning clear and natural.
- Keep the verb in its base form after the modal: "He must be late," not "must is."
- When you're genuinely unsure, reach for might, may or could — a hedged guess is perfectly good English and sounds thoughtful.
- Let context separate obligation-must from deduction-must; you don't need a different word.
❌ Common Mistakes
⚠️ Watch Out
- Negative deduction with mustn't: "He mustn't be home" ❌ → "He can't be home" ✅ (mustn't = forbidden, not impossible).
- Adding to: "She must to be tired" ❌ → "She must be tired" ✅ (no to after must/can't/might).
- Changing the verb for he/she: "It must is broken" ❌ → "It must be broken" ✅.
- Confusing mustn't and don't have to: "You mustn't come, it's optional" ❌ → "You don't have to come" ✅.
📓 Learning Journal
Keep your learning journal going. After this lesson, take a few minutes to write down:
- Key concepts you learned
- Things that clicked for you
- Questions or confusion points to revisit
- Ideas you want to try
- Your progress and feelings about reaching B1
✍️ This lesson's prompt: Look around the room you're in right now and write three deductions about objects or the world outside — one with must, one with can't, and one with might/could — and note the clue behind each. For example: "The window is wet, so it must be raining." Then write one sentence with must as an obligation, and explain in your own words how a listener would know which "must" you mean.
📝 Lesson Summary
🎓 Key Takeaways
- Deduction = modal + base verb: must (sure it's true), can't (sure it's false), might / may / could (maybe).
- The opposite of deduction-must is can't — never mustn't.
- must has two jobs — obligation ("You must wear a seatbelt") and deduction ("You must be tired") — and context tells them apart.
- Obligation family: have to / must = necessary; don't have to = optional; mustn't = forbidden; should = advice.
🎉 What You've Accomplished
You can now do something genuinely sophisticated: reason out loud, signaling exactly how confident you are. That's a huge step up from simply stating facts — it lets you speculate, react to news, solve little mysteries, and sound like a real, thinking speaker. And you've locked down your obligation modals so the whole family is clear. 🎉
❓ Common Questions at This Stage
Is there any difference between might, may and could?
For possibility, they're almost identical — you can usually swap them freely. May sounds slightly more formal (you'll see it in writing and announcements), while might and could are the most common in everyday speech. Pick whichever comes naturally; you won't be wrong.
Can I use can for a positive deduction, like the opposite of can't?
No — that's a neat trap. For a positive deduction we use must ("She must be home"), but the negative is can't ("She can't be home"). The pair is must ↔ can't, not can ↔ can't. Plain can means ability or permission, not certainty.
How do I make a deduction about the past?
You use must have / can't have / might have + past participle ("She must have left already"). It's a natural extension of what you learned today, but it's officially a B2 skill — so just recognize it for now, and keep your own deductions in the present until we teach it fully.
🔭 Looking Ahead
That's the end of Module 4's new grammar. In Lesson 4.3, you'll pull together everything from this module in a Review Project on Opinions & Storytelling — combining your modals, tenses and narrative skills into one connected piece of speaking. It's your chance to see how far you've come.
✅ Before the Next Lesson
- Make five real deductions today about people or things around you, out loud.
- Say the pair aloud until it's automatic: "sure it's true → must; sure it's false → can't."
- Write your Learning Journal entry for this lesson.
🌟 Encouragement for the Journey
Every time you say "it must be…" or "it might be…", you're doing what fluent speakers do without thinking — turning scraps of evidence into ideas, in real time, in English. That's not textbook grammar; that's thinking in your new language. Keep playing detective, keep guessing out loud, and watch how quickly it becomes second nature. See you in the review! 👋