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It's always DNS: must, might and can't for finding the cause

With modals of deduction, students in this B2 English for IT lesson plan work out why photos won't load in one city.

Grammar
Must be, might, may or could be, might not be and can't be for how sure you are, with continuous forms (must be waiting) and the evidence first.
Topic
Photos that won't load after a move to a new CDN, and why engineers joke “It's always DNS”, with words such as hypothesis, symptom and red herring.
Activities
A team chat to read, a diagnosis call to hear, self-checking drills, a chat and mystery boxes, then a troubleshooting message to write and a diagnosis story to record.

What's inside

  1. Chapter 1Warm-up: Something's Broken

    • PictureUnlock
    • Spin the wheel and talk about problems at workSpin the wheelUnlock
    • Study the troubleshooting vocabularyVocabularyTry it
    • Choose the word for each pictureChoose by pictureUnlock

  2. Chapter 2The Story: It Was DNS

    • Read the story of the DNS haikuArticleUnlock
    • Choose the correct answer about the storyQuizUnlock

  3. Chapter 3At Work: Gray Boxes in Chicago

    • Read the thread in the team chatReal-life documentTry it
    • True or false? Check the threadTrue or falseUnlock
    • Choose the conclusion the evidence supportsQuizUnlock

  4. Chapter 4Listen: Diagnose First

    • Listen to Amara and Diego diagnose the problemListeningTry it
    • Transcript, for the teacherUnlock
    • True or false? Listen to the callTrue or falseUnlock

  5. Chapter 5Grammar: It Must Be the Cache

    • Study the scheme: modals of deductionGrammar schemeTry it
    • Choose the correct option to complete each sentenceChoose the optionUnlock
    • Fill in the gaps: rewrite each sentence with a modal of deductionFill in the gapsTry it
    • Drag the words into the gaps in the team chatDrag and dropUnlock
    • Choose how sure you can be from the evidenceQuizUnlock
    • Is the sentence correct? True or falseTrue or falseUnlock

  6. Chapter 6Speak Up: What Must Be Happening?

    • Choose the best replies to DiegoChatTry it
    • Open a box: solve the mysteryOpen the boxUnlock

  7. Chapter 7Homework: Your Troubleshooting Message

    • Read your phrase bankReal-life documentUnlock
    • Choose the correct option to complete Diego's updateChoose the optionUnlock
    • Match the sentence halvesMatchUnlock
    • Write a troubleshooting message with your best guessesWriting, checked by the teacherUnlock
    • Record the story of a problem you diagnosedSpeaking, checked by the teacherUnlock

25 exercises in 7 chapters. Start a 7-day free trial to open all of them, copy any part into your own lessons, and teach the lesson live.

Troubleshooting vocabulary

12 words and phrases from the lesson, each with a picture and audio in edio.

+9 more with pictures and audio in the lessonMore words

  • propagate
  • flush the cache
  • a hypothesis
  • a symptom
  • diagnose
  • a red herring
  • track down
  • a resolver
  • a rule of thumb

Grammar: modals of deduction for troubleshooting

Of the modal verbs, these are the ones for guessing about the present from evidence, and each shows how sure the speaker is, from must be (I'm sure it is) to can't be (I'm sure it isn't). In a troubleshooting thread, learners read the evidence before they pick a modal, add be + -ing for what's happening right now and keep can for what's true in general.

Modals of deduction: how sure are you?

Look at the evidence first. Then choose the modal that matches how sure you are.

From must be to can't be

Four steps of certainty: must be when you're sure it is; might, may or could be when it's possible; might not or may not be when maybe it isn't; can't or couldn't be when you're sure it isn't: It can't be the app: the web dashboard is slow too.

How sure?FormExample
I'm sure it is.must + beThe old address is still coming back, so it must be a cached copy.
It's possible.might / may / could + beIt might be the office Wi-Fi. We haven't checked it yet.
Maybe it isn't.might not / may not + beIt may not be the phone. Let's try another one.
I'm sure it isn't.can't / cannot / couldn't + beIt can't be the app: the web dashboard is slow too.

Happening right now: be + -ing

For an action in progress, the modal takes be + -ing: Her phone must be waiting for the old servers. Some carriers might be holding the old record. New York can't be using the old record. The modal itself never changes: no -s and no to.

ModalContinuous formExample
mustmust be + -ingHer phone must be waiting for the old servers.
might / may / couldmight be + -ingSome carriers might be holding the old record.
might not / may notmay not be + -ingThe tablet may not be showing the new photos yet.
can't / couldn'tcan't be + -ingNew York can't be using the old record.

Evidence first, then the conclusion

Give the evidence, then so, then the modal: The record changed an hour ago, so it must be cached somewhere. The joke line There's no way it's DNS is a deduction too, the same as It can't be DNS, and only the evidence can settle it.

Evidence first, then the conclusion

the evidencesomodal + be (+ -ing)

The record changed an hour ago, so it must be cached somewhere.

New York loads photos from the new CDN, so the CDN can't be down.

From today's lesson

  • There's no way it's DNS. = It can't be DNS.
  • Then it can't be the app. The dashboard doesn't share any code with us.
  • Chicago might still be getting the old DNS record.
  • Any phone that's still using the old record must be waiting for a server that isn't there.
  • His resolver must be using a fresher copy.
  • Restarting servers might not be quick.

Could or can? Can't or mustn't?

For a guess about this case, use could, might or may: It could be the DNS cache. Can means sometimes, in general: DNS can be slow when a resolver is busy. For “I'm sure it isn't”, use can't, because mustn't sounds like a rule.

could or can?

could / might / may: maybe, in this case

  • Photos are gray in Chicago. It could be the DNS cache.

can: sometimes, in general

  • DNS can be slow when a resolver is busy.

must: a rule or a conclusion?

In a spec, must is a rule, and its negative is must not: Passwords must not be shared. In troubleshooting, must is a conclusion, and its usual negative is can't: It can't be the app. Mustn't sounds like a rule, so don't use it for “I'm sure it isn't”. And couldn't means the same as can't here, not “maybe not”.

Typical mistakes

  • It mustn't be DNS: the other sites work fine.

    It can't be DNS: the other sites work fine.

    for “I'm sure it isn't”, use can't

  • Maybe it's the CDN. It can be the CDN.

    It could / might / may be the CDN.

    can = sometimes, in general; for this case, use could / might / may

  • It must to be a cached copy.

    It must be a cached copy.

    modal + base verb, no to

  • It must is the TTL.

    It must be the TTL.

    after a modal, use be, never is / are

More lessons on this grammar: Modal verbs lessons and exercises

Try the modals of deduction exercises

Try them yourself: these are the same exercises your students do in edio.

More exercises

What's in the lesson

  • 13 auto-checked exercises
  • 2 games
  • 2 teacher-checked tasks
  • 2 real-life documents
  • 1 article
  • 1 grammar scheme
  • 1 set of word cards
  • 1 audio clip
  • 1 picture
  • 1 transcript

The main objectives of this lesson are to:

  • say how sure you are about a cause: It must be a cached copy. It might be the office Wi-Fi. It can't be the app.
  • describe what's happening right now with a modal and be + -ing: Her phone must be waiting for the old servers.
  • reason from evidence to a conclusion: New York loads photos from the new CDN, so the CDN can't be down.
  • write a troubleshooting message that shares the evidence and your best guesses before anyone picks a fix

Lesson 11 sits in the middle of Architecture and trade-offs, unit 4 of English for IT B2 · Beyond the Code: before a team chooses a fix, it finds the cause. Lesson 4 used must for a rule in a spec; here it becomes a conclusion, beside might, taught for possibility in English A2.1 lesson 31. Lesson 20 moves the same modals into the past (must have, can't have). Troubleshooting is reasoning from evidence, so each guess needs the modal that matches it.

Twelve troubleshooting words with pictures and audio, from DNS record to red herring, come with a wheel of questions and a picture quiz. The story explains the sysadmins' joke haiku about DNS and why a DNS change reaches some computers late. At work, a team chat follows merchant photos that load as gray boxes in Chicago, with a quiz on the conclusion each clue supports; then a designer and an SRE diagnose it on an AI-voiced call. Five self-checking drills follow the scheme, among them “I'm sure it's…” sentences to rewrite with a modal. Students then reply to Diego in a chat and open boxes of mysteries to solve.

Homework starts from a phrase bank for troubleshooting out loud, then a status update with a modal to choose in each gap and sentence halves to match, both self-checking. Students write a troubleshooting message with their best guesses and record the story of a problem they once diagnosed, for the teacher to review.

Grammar:
modals of deduction · must / might / can't be
Topic:
troubleshooting · DNS · diagnosing problems

Teaching tips

  • Reveal the thread one clue at a time: after each message, the class offers a must, might or can't sentence about the cause, then checks whether the next clue proves it wrong.
  • Draw a certainty line on the board, from must be to can't be, and have students place each guess from the thread and the call on it, with its evidence beside it.
  • In company classes, take a real problem from last week: one learner gives the symptom, the others ask for evidence and answer with It might be… or It can't be… until they agree on a hypothesis.
  • One-to-one, treat the mystery boxes as case studies: the learner reasons aloud about the build that fails only in CI, and you add evidence until it might be becomes it must be.

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The whole course: English for IT B2 · Beyond the Code

24 lessons at B2 for software teams, in American English. A year on at Kestrel Cloud, a fictional payments startup, Iryna and her team estimate work, write docs and postmortems, argue trade-offs, give demos and negotiate scope. Each lesson starts from a real case, from the Apollo 11 alarms to Conway's law.See the course

  1. Unit 1 Planning and estimates

    estimates and deadlines: approximating numbers, the future continuous and perfect, likelihood

    1. Lesson 1It always takes longerApproximating numbers in estimates
    2. Lesson 2By New Year's Eve, we'll have fixed itFuture continuous and future perfect
    3. Lesson 3Let's play planning pokerLikelihood with bound to, likely to and may well

  2. Unit 2 Docs people read

    specs and design docs: modals for requirements, participle clauses, plain English

    1. Lesson 4MUST, SHOULD or MAY?Modal verbs for requirements in specs
    2. Lesson 5Four kinds of docsParticiple clauses in documentation
    3. Lesson 6Just a request for commentsPlain English for design docs

  3. Unit 3 Incidents and postmortems

    telling an incident without blame: narrative tenses, the passive, the third conditional

    1. Lesson 7The 1202 alarmNarrative tenses for incident stories
    2. Lesson 8Blameless by designActive vs passive voice in a postmortem
    3. Lesson 9If we had had an alert…The third conditional: a different past

  4. Unit 4 Architecture and trade-offs

    comparing options and recording decisions: contrast linkers, modals of deduction, mixed conditionals

    1. Lesson 10Your system copies your org chartContrast linkers for comparing options
    2. Lesson 11 · you're hereIt's always DNSModals of deduction for troubleshooting
    3. Lesson 12Write down the whyMixed conditionals for looking back on decisions

  5. Unit 5 Demos and tech talks

    a clear structure on stage: signposting, cleft sentences, describing trends

    1. Lesson 13The mother of all demosSignposting language for demos
    2. Lesson 1420 slides, 15 secondsCleft sentences for emphasis
    3. Lesson 15Numbers that tell a storyDescribing graphs and trends

  6. Unit 6 Stakeholders and scope

    saying no and finding yes: diplomatic language, as long as and unless, negotiating

    1. Lesson 16Pay down the debtDiplomatic language for hard messages
    2. Lesson 17Why meetings argue about the bike shedAs long as, provided, unless and otherwise
    3. Lesson 18Yes, if…Negotiating language for scope and deadlines

  7. Unit 7 AI, security and ethics

    AI tools, passwords and team rules: be used to and get used to, past modals of deduction, purpose linkers

    1. Lesson 19Your new pair programmerUsed to, be used to and get used to
    2. Lesson 20Change your password? Maybe notPast modals of deduction
    3. Lesson 21Rules for AI at workPurpose linkers for rules with a reason

  8. Unit 8 Growing your career

    feedback, retros and goals: phrasal verbs, reporting verbs, wish and if only

    1. Lesson 22Your next levelCareer phrasal verbs and goal frames
    2. Lesson 23Feedback that helpsReporting verbs and their patterns
    3. Lesson 24Start, stop, continueWish and if only for wishes and regrets

Questions about this lesson

What does "it must be" mean?
It must be means “I'm sure it is”, a conclusion from evidence, not a rule: The old address is still coming back, so it must be a cached copy. For an action happening now, use must be + -ing: Her phone must be waiting for the old servers. After must, the verb is be: It must be the TTL, never It must is or It must to be.
When do you use might be instead of must be?
Use might be when a cause is possible but not proven yet: It might be the office Wi-Fi. We haven't checked it yet. May be and could be work the same way. Switch to must be once the evidence leaves one explanation: Chicago might still be getting the old DNS record is a hypothesis, but after a lookup returns the old address, the resolver must be using a cached copy.
What are modals of deduction?
Modals of deduction are modal verbs that show how sure you are about something you can't check directly: must be (I'm sure it is), might, may or could be (it's possible), might not be (maybe it isn't) and can't be (I'm sure it isn't). They take the base verb and never change for the person. The B2 lesson It's always DNS on edio.ai uses them to diagnose photos that won't load.
Are modals of speculation the same as modals of deduction?
Mostly, yes: many coursebooks use both names for one set of modals, must, might, may, could and can't, for guesses about the present. When a book separates them, deduction is a conclusion from evidence (It can't be the app: the dashboard is slow too), and speculation is a guess with little evidence yet (It might be his phone). The same scale works in the past: must have, might have, can't have.
Is it "it mustn't be" or "it can't be"?
For “I'm sure it isn't”, say it can't be, not it mustn't be: It can't be DNS: the other sites work fine. Mustn't sounds like a rule, as in a spec (Passwords must not be shared), so a listener hears an order, not a conclusion. Couldn't be means the same as can't be here, and for maybe not, use might not or may not: It may not be the phone.
Can you say "It can be the CDN" when you're guessing?
No: for one particular case, use could, might or may, as in Photos are gray in Chicago. It could be the DNS cache. Can describes what is sometimes true in general: DNS can be slow when a resolver is busy. So It can be the CDN sounds like a general fact about CDNs, not a guess about today. A true-or-false drill in It's always DNS on edio.ai tests exactly this pair.
What does "it's always DNS" mean?
“It's always DNS” is an engineers' joke about how often DNS is the culprit, from a haiku nobody can credit: It's not DNS. There's no way it's DNS. It was DNS. DNS answers are cached, so a changed record can look right on one computer and wrong on another. People say it half-joking when a strange problem turns out to be DNS, but it's a rule of thumb, not a law.
What is a red herring when you troubleshoot?
A red herring is a clue that leads you away from the real cause, often a recent change that looks guilty: I think the image-loading change is a red herring. Rule it out with evidence (The dashboard doesn't share any code with us, so it can't be the app), then name the real suspect as a hypothesis, not yet a diagnosis. The team chat in It's always DNS on edio.ai has one to spot.
How do you share your best guess about a problem at work?
Describe the symptom, give the evidence, then guess: We're seeing gray boxes in Chicago. It started after the move last night. It must be a cached copy. Say what you've ruled out and why (It can't be the app, because the web is slow too), and end with the next check: Before we choose a fix, let's check the TTL. The phrase bank of It's always DNS on edio.ai collects these lines.
How do I teach modals of deduction to B2 adults?
Give the evidence before the grammar: a symptom and three clues, then ask how sure the class is. Map the answers onto a line from must be to can't be, add be + -ing for what's happening now, and only then drill the traps, mustn't and It can be. Working adults can bring their own mysteries, such as a build that passes on every laptop but fails in CI.
What's good homework for practicing modals of deduction?
Good homework is a troubleshooting message that offers guesses, not a fix: the symptom, the evidence, what it can't be and the best guesses with must be, might be and one continuous form. In It's always DNS on edio.ai, students write that message and record how they once diagnosed a problem, both for the teacher to review; a status update with modals to choose and a sentence match check themselves.
Are there interactive exercises for must, might and can't?
Yes: rewrite drills and chats that check the modal show at once whether a guess is too sure or not sure enough. This page plays four parts of It's always DNS, a B2 English for IT lesson on edio.ai, without an account: the team chat thread about gray photos, a diagnosis call, a gap-fill that rewrites each sentence with a modal of deduction, and a chat with Diego. The other drills are locked.
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