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Grammar
Modality B1 B2

Modal deduction and speculation

Judge present and future certainty with must, can't, may, might, could, and should.

Visual guide

Modal deduction: the certainty scale

Evidence → choose the helper by certainty

IMPOSSIBLE       POSSIBLE          EXPECTED       ALMOST CERTAIN
can't            may / might /     should         must
                 could
   │                │                │              │
That can't be our taxi; the number is different.
They might be in the café.
The train should arrive soon.
The lights are on, so Ben must be home.

Shape:
person/thing + certainty helper + be / plain verb

✓ She must be tired.   ✗ She must tired.

Choose the modal by how sure the evidence makes you, then use it before be or the plain verb.

Introduction

Judging from evidence

Modal deduction and speculation help you judge what is true when you do not know for sure. You look at evidence, the situation, or normal expectations, then choose a modal to show your certainty. The lights are on, so Ben must be home. That can't be our taxi; the number is different. These sentences do not report direct facts; they report the speaker's present judgement about a present situation or a likely future result.
Core Grammar B1 Vocabulary A2 Neutral

The lights are on, so Ben must be home.

Must shows a strong logical conclusion, not obligation, because the evidence points to it.

Building blocks

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The lights are on, so
evidence_frame
Gives the reason for the conclusion.
Ben must be home
strong_deduction
Shows a strong present conclusion from evidence.
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Negative Grammar B1 Vocabulary A2 Neutral

That can't be our taxi; the number is different.

Can't expresses a negative deduction: the speaker is sure something is not true.

Building blocks

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That can't be our taxi
impossible_deduction
Shows that the speaker thinks this possibility is impossible.
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the number is different
evidence_frame
Gives the evidence for rejecting the idea.
Cognitive metaphor

A scale of certainty

Think of these modals as points on a certainty scale. Must is very strong: the evidence points clearly to one conclusion. Should means expected or likely because it is normal, planned, or reasonable. May, might, and could are in the middle: possible, but not claimed as certain. Can't is very strong in the opposite direction: the speaker thinks the idea is impossible. She must be the new manager. The train should arrive soon. This can't be the right key. They might be in the café.
Core Grammar B1 Vocabulary A2 Neutral

She must be the new manager.

Must be is the normal frame for a strong conclusion about identity or state.

Building blocks

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She must be the new manager
strong_deduction
A strong conclusion about who she is.
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Core Grammar B1 Vocabulary A2 Neutral

The train should arrive soon.

Should often means the speaker expects something because it is normal, planned, or likely.

Building blocks

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The train should arrive soon
expected_result
Shows an expected result, but not complete certainty.
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Negative Grammar B1 Vocabulary A2 Neutral

This can't be the right key.

Can't is the opposite of must for deduction: the speaker feels sure the idea is wrong.

Building blocks

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This can't be the right key
impossible_deduction
Rejects the possibility as logically impossible or very unlikely.
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Core Grammar B1 Vocabulary A2 Neutral

They might be in the café.

Might be is useful when the speaker has a real possibility but weak evidence.

Building blocks

Read the sentence in chunks, not word-by-word.

They might be in the café
weak_possibility
Shows a possible present location without strong certainty.
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Core pattern

The basic pattern

The core form is subject + modal + base verb. The modal does not add -s, -ed, or -ing, and the following verb stays in its base form. With adjectives, nouns, and places, use be after the modal. She must tired is a common mistake; She must be tired is correct. For future actions, use the same modal + base verb pattern: They may arrive early. For possible results, could is very common in speech: This could work.
Mistake Grammar B1 Vocabulary A1 Neutral

She must tired.

After a modal, English needs the base verb. With an adjective like tired, use be.

Corrected Grammar B1 Vocabulary A1 Neutral

She must be tired.

Use modal + be + adjective for deductions about states.

Building blocks

Read the sentence in chunks, not word-by-word.

She must be tired
strong_deduction
A reusable frame for a strong conclusion about someone's state.
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Core Grammar B1 Vocabulary A2 Neutral

They may arrive early.

May + base verb can speculate about a future action.

Building blocks

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They may arrive early
future_possibility
Shows a possible future event.
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Spoken Grammar B1 Vocabulary A2 Neutral

This could work.

Could often sounds natural for suggesting that something is possible.

Building blocks

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This could work
possible_result
A short reusable frame for a possible solution or result.
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Form variations

Useful form variations

Deduction often uses be because we are judging a state, identity, place, or situation. Use modal + be + adjective, noun, or place for states and identities. Use modal + be + -ing for an action possibly happening now. Use modal + be + past participle for passive meanings. It might be raining downtown. The road may be closed. The report should be ready by noon. These patterns let you speculate about now, soon, or a future result without changing the modal form.
Core Grammar B2 Vocabulary A2 Neutral

It might be raining downtown.

Use modal + be + -ing for speculation about an action in progress.

Building blocks

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It might be raining
continuous_speculation
Shows a possible action happening now.
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downtown
place_context
Adds the place where the situation may be happening.
Core Grammar B2 Vocabulary A2 Neutral

The road may be closed.

Use modal + be + past participle when the subject receives an action or is in a passive state.

Building blocks

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The road may be closed
passive_speculation
Shows a possible passive state.
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Formal Grammar B1 Vocabulary A2 Formal

The report should be ready by noon.

Should be ready is common for expected completion, especially in work or service contexts.

Building blocks

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The report should be ready
expected_ready_state
Shows an expected state.
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by noon
deadline
Gives the expected time limit.
Usage rules

Negatives and questions

Negative forms do not all mean the same thing. May not and might not mean that a negative situation is possible: the speaker is not sure. Can't means the speaker is sure the idea is not true. She may not know the address. He can't be asleep; his light is on. In questions, could is the most common neutral choice for asking about a possible explanation. Could this be a mistake? Might they need more time? Might sounds more careful or formal than could.
Negative Grammar B1 Vocabulary A2 Neutral

She may not know the address.

May not means possible non-truth, not impossibility.

Building blocks

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She may not know
negative_possibility
Shows that it is possible she does not know.
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the address
object
Says what she may not know.
Negative Grammar B1 Vocabulary A2 Neutral

He can't be asleep; his light is on.

Can't is much stronger than may not or might not.

Building blocks

Read the sentence in chunks, not word-by-word.

He can't be asleep
impossible_deduction
Shows the speaker is sure the idea is false.
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his light is on
evidence_frame
Gives the evidence for the deduction.
Question Grammar B1 Vocabulary A2 Neutral

Could this be a mistake?

Could is common and natural in questions about possible explanations.

Building blocks

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Could this be a mistake
possibility_question
Asks whether something is possible.
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Question Grammar B2 Vocabulary A2 Formal

Might they need more time?

Might can form speculation questions, especially when the speaker is cautious.

Building blocks

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Might they need more time
possibility_question
Asks about a possible need, often a little more careful or formal than could.
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Contrast

Deduction is not obligation

The closest confusion is must for deduction versus must for obligation. You must be cold means I conclude this from what I see or know. You must wear a coat means I am giving a rule or strong instruction. The verb pattern and situation decide the meaning. Another important contrast is can't versus mustn't. Can't can reject a possibility in deduction. Mustn't means prohibition, so it is usually a rule or warning, not a logical conclusion.
Contrast Grammar B1 Vocabulary A1 Neutral

You must be cold.

Here must is deduction because it is followed by be + adjective and fits the evidence.

Building blocks

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You must be cold
deduction_not_obligation
Shows a conclusion about someone's state, not a command.
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Contrast Grammar B1 Vocabulary A1 Neutral

You must wear a coat.

Must can also express obligation; context and verb meaning decide the reading.

Building blocks

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You must wear a coat
obligation
Shows a rule or command, not a deduction.
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Danger zone

Common boundaries

Do not use can for a single uncertain future guess in ordinary English. It can rain in April means rain is generally possible in April. For one specific future situation, use may, might, or could: It might rain tonight. Also remember that these modals normally describe present or future judgement in this lesson. For past deduction, English uses forms such as must have left or might have forgotten; those are a separate pattern and should not be mixed with the simple present/future frames here.
Exception Grammar B1 Vocabulary A1 Neutral

It can rain in April.

Can is natural for general possibility, but not usually for a single uncertain future event.

Building blocks

Read the sentence in chunks, not word-by-word.

It can rain
general_possibility
Shows something that is generally possible, not a one-time guess.
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in April
general_time_context
Marks the sentence as a general statement about a season or period.
Summary

The practical choice

Choose the modal by asking how strong your evidence is. Use must when the evidence strongly points to yes. Use can't when the evidence strongly points to no. Use may, might, or could when something is only possible. Use should when something is expected because it is normal, planned, or likely. Keep the form simple: modal + base verb, modal + be + adjective/place/noun, modal + be + -ing, or modal + be + past participle. The modal shows your judgement, not a guaranteed fact.
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