English Grammar Lab

← B2

Must, might and can't for deduction

You will be able to Express certainty, possibility and impossibility.

Explanation

Use must for a strong conclusion, might for possibility and can't for an impossible conclusion.

Examples

Detailed course

Understand the grammar point

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Detailed explanation

This lesson focuses on must, might and can't for deduction. The structure helps learners sequence complex events, and it should always be interpreted as part of a complete message rather than as an isolated formula. Before selecting a form, identify the subject, time reference, clause relationship, and the information that receives emphasis.

Formation is central. Use must plus the base form for a strong logical conclusion, might, may, or could for possibility, and can’t for a conclusion that something is impossible. Build the sentence in stages: identify the grammatical subject, choose the auxiliary or main verb form, and complete the clause in normal English word order. In negatives and questions, check whether the structure supplies its own auxiliary or requires another one.

The pattern can help users express degrees of certainty, compare real and hypothetical outcomes, and compress information without losing clarity. These purposes may overlap, so context decides which interpretation is natural. Time expressions, determiners, linking words, and punctuation often provide evidence for the correct choice.

A useful contrast prevents overgeneralisation. Must for deduction expresses a conclusion, while must for obligation expresses a requirement. Ask how the nearest alternative would change time, certainty, focus, politeness, or register. A grammatically possible sentence may still communicate the wrong relationship if that contrast is ignored.

The guided examples are complete messages. Notice how the target structure agrees with its subject and how the surrounding vocabulary makes the intended meaning clear. When producing a new example, preserve the relationship between form and meaning while changing the people, place, and lexical details.

A reliable self-check is to paraphrase the sentence in simpler language. If the paraphrase changes the time relationship, degree of certainty, or focus, review the original form. Compare the sentence with the common mistakes and explain why the corrected version is preferable.

In connected speech and writing, the structure also contributes to cohesion. It guides the reader through background information, main events, causes, qualifications, and results. Clause order and connector choice can therefore affect interpretation even when every individual word is correct.

Register matters as well. A complex construction may be appropriate in a report but unnecessarily heavy in conversation, while an informal alternative may be too vague for academic writing. Choose the form because it serves the communicative purpose, not because it merely sounds advanced.

How to form it

Use must plus the base form for a strong logical conclusion, might, may, or could for possibility, and can’t for a conclusion that something is impossible.

Use the structure to sequence complex events.

Use the structure to express degrees of certainty.

Use the structure to compare real and hypothetical outcomes.

Use the structure to compress information without losing clarity.

Must for deduction expresses a conclusion, while must for obligation expresses a requirement.

Guided examples

The office lights are on, so someone must be inside.

Must expresses a strong, confident conclusion based on the evidence, the lights.

The missing file might be in the archive room.

Might expresses a possibility, not a strong conclusion.

The director can’t be in two meetings at the same time.

Can’t expresses that a conclusion is impossible, based on logic.

This key could belong to the maintenance team.

Could expresses a possibility, similar in strength to might.

They must know about the schedule change by now.

Must expresses confidence that something is true based on the situation.

The noise may be coming from the ventilation system.

May expresses a possible explanation, without full certainty.

Common mistakes

Not: The office lights are on, so someone must to be inside.

Use: The office lights are on, so someone must be inside.

Must is followed directly by the base verb, without to.

Not: The missing file might to be in the archive room.

Use: The missing file might be in the archive room.

Might is followed directly by the base verb, without to.

Not: The director mustn’t be in two meetings at the same time.

Use: The director can’t be in two meetings at the same time.

Can’t expresses impossibility here; mustn’t would express prohibition, which is not the meaning intended.

Not: This key could belongs to the maintenance team.

Use: This key could belong to the maintenance team.

After could, the verb stays in its base form: belong, not belongs.

Before you practise

  • Identify the communicative purpose.
  • Check the subject and required verb or auxiliary.
  • Confirm the time reference and clause order.
  • Compare the sentence with the nearest alternative.
  • Read the complete sentence and verify the meaning.

Reading text

The Open Office Door

About 383 words

Iris, the cleaner, a designer, and the manager met at a quiet design office before opening time to work out why one office door is open. What first looked like a small problem became more complicated because no one can immediately explain the open door. The group placed lights, a coffee cup, a missing coat, and a meeting-room screen on the table and built a timeline from the earliest clue to the last decision. The cleaner must be nearby because the lights are on. That discovery forced them to question the first version of events. The coffee cup might belong to the early designer. Instead of blaming one person, they compared the evidence and marked every uncertain point. The manager cannot be in her office because her coat is missing. A second check showed that one assumption had been too quick. The meeting room screen could be updating. Once the timeline was complete, the problem no longer looked random. The notes showed which choices mattered, which details were only distractions, and why the result had been so frustrating. By closing time, the result was clear: the team checks the clues before calling the manager. Iris, the cleaner, a designer, and the manager return to lights, a coffee cup, a missing coat, and a meeting-room screen because each item helps with the task. The notes show why no one can immediately explain the open door. The group checks the time, the place, and the next decision before moving on. No one rushes, because a small error would change the outcome. The work stays connected to a quiet design office before opening time, where the problem first became visible. By the end, the result is clear: the team checks the clues before calling the manager. Later, the written record helps the group explain the decision without adding new claims. The strongest clue comes from the materials, not from anyone's memory alone. Several details point in different directions, so the group keeps the conclusion careful. The final record can be checked again if the question returns. Iris, the cleaner, a designer, and the manager return to lights, a coffee cup, a missing coat, and a meeting-room screen because each item helps with the task. The notes show why no one can immediately explain the open door.

Reading comprehension

  1. Who is involved in the reading?

    Show answer

    Iris, the cleaner, a designer, and the manager

  2. Where does the reading take place?

    Show answer

    a quiet design office before opening time

  3. What task are the people trying to complete?

    Show answer

    work out why one office door is open

  4. What problem makes the task less simple?

    Show answer

    no one can immediately explain the open door

  5. How does the reading end?

    Show answer

    the team checks the clues before calling the manager

Grammar in the text

  1. The cleaner must be nearby because the lights are on.

    Must expresses a strong conclusion based on the evidence given.

  2. The coffee cup might belong to the early designer.

    Might expresses a possible, less certain explanation.

  3. The manager cannot be in her office because her coat is missing.

    Cannot expresses that this conclusion is logically impossible.

  4. The meeting room screen could be updating.

    Could expresses a possible explanation for the situation.

Key point

Use this pattern when the meaning needs it in the reading: must/might/could/cannot + base verb. In short, modal verbs show different levels of certainty about the present. Example: "The cleaner must be nearby because the lights are on."

Practice

  1. Complete the sentence from the same situation: "The cleaner ____ nearby because the lights are on."

    Show answer

    The cleaner must be nearby because the lights are on.

  2. Complete the sentence from the same situation: "The coffee cup ____ to the early designer."

    Show answer

    The coffee cup might belong to the early designer.

  3. Complete the sentence from the same situation: "The manager ____ in her office because her coat is missing."

    Show answer

    The manager cannot be in her office because her coat is missing.

  4. Complete the sentence from the same situation: "The meeting room screen ____ updating."

    Show answer

    The meeting room screen could be updating.

Listening practice

Watch a TEDx talk

“The power of introverts” — Susan Cain (TED). Listen for clear, natural English and pick up new vocabulary while you review this grammar point.

Check your understanding

What does Susan Cain say Western culture, especially in the United States, tends to favor?

Check your understanding

Which of these is one of Susan Cain's three calls to action at the end of her talk?

Practice

Choose the correct answer. The lights are on. Someone ___ be home.

Practice

Choose the correct answer. I am not certain, but he ___ know her.

Practice

Choose the correct answer. That ___ be Paul. He is abroad.

Written practice

Produce the answer yourself. Capitalisation and final punctuation do not affect validation, but the grammar and word order must be correct.

Sentence correction

Written practice

Rewrite the sentence to express a strong logical conclusion using "must".

She can't be tired after working twelve hours.