Possibility and deduction
Possibility leaves alternatives open; deduction evaluates evidence and places a claim on a scale from uncertain to nearly certain.
01 · Concept foundation
Understand the terms before applying the rule
Each term below names a different grammatical object. Open examples and compare their function rather than memorising a Vietnamese translation alone.
epistemic modality/ˌepɪˈstemɪk məʊˈdæləti/
tình thái nhận thứcModality expressing the speaker's judgement about the truth or likelihood of a proposition.
The error may be systematic.
Sai số có thể mang tính hệ thống.
deduction/dɪˈdʌkʃən/
suy đoán logicA conclusion drawn from available evidence rather than direct knowledge.
The sensor must be faulty.
Cảm biến hẳn là bị lỗi.
modal perfect/ˈməʊdəl ˈpɜːfekt/
modal hoàn thànhModal + have + past participle, used to evaluate a past possibility, deduction or unrealized action.
The sensor might have failed.
Cảm biến có thể đã hỏng.
Complete lesson scope
Do not stop at one formula
May, might and could for possibility
Must, can't and couldn't for logical deduction
Present, progressive and perfect deductions
Degrees of certainty and evidence
Decision boundary: Must not expresses prohibition, whereas can't in deduction expresses logical impossibility.
02 · Controlling rule
May, might and could keep a proposition open as possible; must expresses a strong positive inference from evidence; can't/couldn't expresses logical incompatibility. Can often describes general potential, whereas may/might normally evaluates a specific situation. Combine the modal with be + V-ing for an ongoing process and have + past participle for a past event. In academic writing, the modal should match the evidence and work with specialised verbs such as indicate, suggest, reflect, arise from and contribute to.
may/might/could/must/can't + be/V | modal + have + V3Possibility, probability, deduction and evidential stance
Match the strength of a claim to the available evidence and distinguish open possibility, general possibility, strong deduction and logical impossibility in present, ongoing and past situations.
Epistemic modality calibrates commitment to truth
A modal is not a numerical probability label. May, might and could overlap, while must and cannot express conclusions from evidence. Context, genre and the quality of evidence determine how the reader interprets the strength.
strength of evidence
source of evidence: observation, report, inference or prior knowledge
time of the proposition: present, ongoing or past
rhetorical risk of overclaiming or underclaiming
Open possibility: may, might and could
These modals present a proposition as possible rather than established. Might is often more tentative, but the difference is contextual rather than a fixed percentage scale.
may/might/could + base verbUse may/might/could when several explanations remain compatible with the evidence.
Use may not/might not for negative possibility; do not use must not unless prohibition is intended.
Could often highlights one possible mechanism or outcome among alternatives.
The observed delay may reflect a boundary-condition error.
The observed delay may reflect a boundary-condition error.
May qualifies the interpretation; reflect names the evidential relationship.
plausible but unconfirmedThe discrepancy could result from an incorrect datum.
The discrepancy could result from an incorrect datum.
Could introduces one causal hypothesis without claiming it is the only cause.
Present deduction
The device must be faulty.The device can't be faulty.Could the device be faulty?- Use be for states and identity.
- Must and can't express conclusions, not direct observation.
Past deduction
The device must have failed.The device can't have failed.Could the device have failed?- Use have + past participle after the modal.
- The assessment is current even though the event is past.
The selected form changes commitment and social force
The file is corrupt.
The writer takes full responsibility for the proposition within the stated evidence frame.
The file must be corrupt.
The evidence strongly supports the conclusion, but it remains inferred.
The file may be corrupt.
The conclusion is compatible with the evidence but not confirmed.
The file might be corrupt.
The speaker marks greater distance from the conclusion.
The file can't be corrupt.
The evidence is treated as incompatible with the proposition.
Inference versus fact
The sample is contaminated.
direct claim
The sample must be contaminated.
strong evidence-based inference
Use must only when the sentence is a conclusion from evidence, not when contamination was directly confirmed.
Prohibition versus impossibility
You mustn't enter.
entry is prohibited
He can't be inside.
being inside is logically impossible
Mustn't is deontic; can't in deduction is epistemic.
Specific possibility versus general potential
Heavy rainfall may cause flooding tonight.
one possible future event
Heavy rainfall can cause flooding.
general causal potential
Use can for what is generally possible and may/might for what may happen in this specific case.
IELTS Speaking and discussion
- Prefer
- I think it may/might..., It could be because..., It must be...
- Avoid
- repeated definite will for opinions
- Why
- A range of calibrated forms shows control of certainty and reasoning.
Academic argument
- Prefer
- may indicate, might suggest, could reflect, is likely to
- Avoid
- must/will without explicit evidence
- Why
- The wording should expose the inferential status of the claim.
Results and discussion sections
- Prefer
- results show..., this may be explained by..., the pattern is consistent with...
- Avoid
- causes when only association was measured
- Why
- Modal and evidential verb choice protects the distinction between observation, association and causal explanation.
Let the modal control force and the lexical verb control precision
Offer an interpretation
may/might/could + indicate/suggest/imply/reflectThe asymmetry may indicate a tidal-phase error.
The verb specifies how the observation supports the interpretation.
Propose a causal mechanism
may/could + result from/arise from/be caused byThe oscillation could arise from numerical dispersion.
Use a modal when the mechanism has not been isolated experimentally.
State a qualified prediction
may/might/is likely to + increase/decrease/remain/exceedPeak salinity is likely to increase during prolonged low flow.
Choose a trend verb that names the expected direction.
Certainty and evidence map
The scale is conceptual, not a numerical probability conversion.
| Expression | Typical meaning | Evidence relation |
|---|---|---|
| must | strong positive deduction | evidence strongly converges |
| may/might/could | open possibility | evidence is compatible but incomplete |
| can't/couldn't | logical impossibility | evidence conflicts with proposition |
| unmodalised statement | asserted fact | writer assumes responsibility for truth |
Time and aspect in deduction
The modal expresses current stance; the chain expresses event time and aspect.
| Meaning | Form | Example |
|---|---|---|
| present state | must/may/can't + be | The datum may be wrong. |
| ongoing process | modal + be + V-ing | The pump may be cavitating. |
| completed past event | modal + have + V3 | The sensor must have failed. |
| past passive event | modal + have been + V3 | The file may have been replaced. |
✕ The reading mustn't be correct; it conflicts with every other sensor.
✓ The reading can't be correct; it conflicts with every other sensor.
Negative deduction uses can't/couldn't; mustn't normally means prohibition.
✕ The new policy will reduce pollution, in my opinion.
✓ The new policy may reduce pollution, in my opinion.
When the outcome is uncertain, may/might/could calibrates the opinion more accurately than definite will.
✕ The sensor may failed during the storm.
✓ The sensor may have failed during the storm.
Past possibility requires modal + have + past participle.
✕ The result may possibly perhaps indicate a bias.
✓ The result may indicate a bias.
Stacked hedges weaken clarity without adding a precise evidential distinction.
Choose by meaning, evidence and relationship
1. Which sentence expresses strong positive deduction?
2. Which form expresses a possible past event?
3. Which sentence states general causal potential?
4. Which sentence avoids overclaiming an unproven cause?
Write a five-sentence interpretation of one result: one observation, two alternative explanations, one strong deduction and one rejected explanation. Name the evidence supporting each level of certainty.
Direct observation is separated from inference.
May/might/could is used where evidence remains incomplete.
Must or can't is supported by explicit converging or conflicting evidence.
Past or ongoing deductions use the correct auxiliary chain.
03 · Worked examples
Observe form, function and meaning together
The observed delay may reflect a boundary-condition error.
Độ trễ quan trắc có thể phản ánh lỗi điều kiện biên.
All channels show the same offset, so the datum must be wrong.
Mọi kênh đều có cùng độ lệch nên mốc cao độ hẳn bị sai.
The pump may be cavitating under the current load.
Máy bơm có thể đang bị xâm thực dưới tải hiện tại.
The sensor can't be disconnected because it is still transmitting data.
Cảm biến không thể đang bị ngắt kết nối vì nó vẫn truyền dữ liệu.
04 · High-risk contrast
Explain why one form fails, not only which answer is correct
The reading mustn't be correct because every other sensor disagrees.
The reading can't be correct because every other sensor disagrees.
Negative deduction uses can't/couldn't. Mustn't normally means that an authority prohibits an action.
05 · Mastery check
Apply the rule before marking the lesson complete
Which sentence is grammatically acceptable in the target system?
Which description best defines “epistemic modality”?
Which example is one of the verified target patterns in this lesson?
Which structural formula belongs to this lesson?
Complete all four checks, then submit a sentence for target-form feedback.
06 · IELTS Academic
Transfer grammar into a real communicative task
Use calibrated modals to avoid unsupported certainty in Task 2 and discussion sections: may indicate, might suggest, could reflect, must be distinguished from and is likely to increase. Keep direct observations separate from inference, and do not replace evidence with a decorative hedge.
Explain how the selected modal changes truth commitment or social force.
Build affirmative, negative, question, perfect, progressive or passive forms without breaking the auxiliary order.
Distinguish two forms that can describe the same event but imply different evidence, authority or politeness.
Use a specialised verb that makes the proposed action or inference operationally precise.