Core modal form
Core modal auxiliaries express the speaker's judgement and are followed by the bare infinitive without to or third-person -s.
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.
modal auxiliary/ˈməʊdəl ɔːɡˈzɪliəri/
trợ động từ khiếm khuyếtAn auxiliary expressing meanings such as possibility, ability, obligation, deduction or stance.
can, could, may, might, must, should, will, would
can, could, may, might, must, should, will, would
bare infinitive/beə ɪnˈfɪnətɪv/
động từ nguyên mẫu không toThe base form of a verb used without to after core modal auxiliaries.
may increase, should report
có thể tăng, nên báo cáo
modality/məʊˈdæləti/
tình tháiThe grammatical expression of a speaker's assessment of possibility, necessity, permission, willingness or obligation.
The result may change.
Kết quả có thể thay đổi.
Complete lesson scope
Do not stop at one formula
Core modal + bare infinitive
Negatives, questions and short answers with modals
Modal perfect, progressive and passive combinations
Semi-modals such as have to, be able to and be supposed to
Decision boundary: Core modals do not take third-person -s, do-support or an immediate to-infinitive.
02 · Controlling rule
Core modals are finite auxiliaries followed by the bare infinitive. They do not take third-person -s, do not use do-support and can combine with perfect, progressive and passive auxiliaries in the fixed order modal + have + be + lexical verb. The modal contributes the speaker's assessment—certainty, willingness, authority or necessity—while the remaining chain locates and profiles the event.
subject + modal + (not) + have + be + lexical verb formCore modal architecture: form, scope and stance
Build modal verb phrases accurately and understand that a modal does not merely add grammar: it places the speaker between a proposition and reality by marking certainty, willingness, authority, necessity or social attitude.
Modality = proposition + speaker assessment
The same event can be framed as a fact, a possibility, a recommendation or an obligation. Choosing the modal therefore changes both truth commitment and the relationship between speaker and listener.
epistemic certainty: how certain is the claim?
deontic: what is required, permitted or prohibited?
dynamic: what can the subject do?
interpersonal authority: how direct, polite or authoritative is the utterance?
Core modal syntax
Core modals are finite auxiliaries. They carry tense or stance, precede the lexical verb, do not take third-person -s and form questions or negatives without do-support.
subject + modal + (not) + base verbUse the bare infinitive after can, could, may, might, must, shall, should, will and would.
Invert the modal and subject in a direct question: Could the model fail?
Place not after the modal: may not, should not, cannot. Meaning depends on the modal, not only on not.
The revised model may reduce the bias.
The revised model may reduce the bias.
May scopes over reduce and presents the result as possible, not guaranteed.
moderate epistemic commitmentShould the team repeat the calibration?
Should the team repeat the calibration?
Should moves before the subject because it is the finite auxiliary in a direct question.
Core modal
The model may fail.The model may not fail.May the model fail?- No third-person -s and no immediate to-infinitive.
- The modal carries finiteness; the lexical verb stays in the base form.
Modal perfect passive
The data may have been corrupted.The data may not have been corrupted.Could the data have been corrupted?- Keep the order modal + have + been + past participle.
- The modal gives the current stance; the perfect locates the event before now.
The selected form changes commitment and social force
The model is unstable.
The speaker presents the proposition as a fact within the stated frame.
The model must be unstable.
Evidence strongly supports the conclusion, but the sentence remains an inference.
The model may be unstable.
The proposition is plausible but not confirmed.
The model might be unstable.
The speaker creates more distance from the claim.
Fact versus assessment
The method is reliable.
direct assertion
The method may be reliable.
qualified possibility
Use a modal when the evidence or rhetorical purpose does not justify an unqualified factual claim.
Negative possibility versus prohibition
The device may not work.
perhaps it will fail
You must not use the device.
use is prohibited
Do not choose a negative modal from translation alone; identify whether the intended relation is probability or authority.
Core modal versus semi-modal
The team must leave now.
current strong obligation
The team had to leave early.
past external necessity
Choose a semi-modal when tense, aspect or person marking is required and the core modal cannot supply it naturally.
Everyday conversation
- Prefer
- can, could, should, have to, might
- Avoid
- overly dense auxiliary chains without need
- Why
- Conversation favours quick, interpretable stance marking.
Academic writing
- Prefer
- may indicate, might reflect, can be explained by, must be distinguished from
- Avoid
- unqualified will/must when evidence is limited
- Why
- Academic claims must match the strength and source of the evidence.
Technical procedure
- Prefer
- must, shall, is required to, should
- Avoid
- ambiguous may where compliance is mandatory
- Why
- Standards need a controlled distinction between requirement, recommendation and permission.
Let the modal control force and the lexical verb control precision
Qualify evidence-based claims
may/might/could + indicate/suggest/reflect/result fromThe discrepancy may indicate an unmodelled boundary effect.
These verbs state the evidential relationship more precisely than a vague modal alone.
State procedural control
must/shall + comply/submit/verify/ensureThe operator must verify the datum before calibration.
Choose a verb that names the required action, not a generic do or make.
Make a measured recommendation
should/could + consider/review/compare/evaluateThe study should compare at least two calibration periods.
The modal controls force; the specialised verb identifies the analytical operation.
Fixed order in modal verb phrases
Not every sentence uses every slot, but the relative order does not change.
| Slot | Function | Example |
|---|---|---|
| 1 | modal | might |
| 2 | perfect have | have |
| 3 | progressive/passive be | been |
| 4 | lexical verb form | affected |
Four functional domains
A single modal can belong to more than one domain; context resolves the meaning.
| Domain | Question | Typical expressions |
|---|---|---|
| epistemic | How certain? | may, might, must, can't |
| deontic | What is required/allowed? | must, may, should |
| dynamic | What can the subject do? | can, could, be able to |
| interpersonal | How direct/polite? | could, would, shall |
✕ The model may to underestimate the peak.
✓ The model may underestimate the peak.
A core modal is followed directly by the base form.
✕ The model cans reproduce the trend.
✓ The model can reproduce the trend.
Core modals do not take third-person -s.
✕ Does the model can reproduce the trend?
✓ Can the model reproduce the trend?
A core modal itself forms the question; do-support is not used.
✕ The data might been have corrupted.
✓ The data might have been corrupted.
The auxiliary order is modal + have + been + past participle.
Choose by meaning, evidence and relationship
1. Which sentence has a valid core-modal form?
2. Which order forms a modal perfect passive?
3. What changes when “is” becomes “may be”?
4. Which structure supplies past necessity?
Write a four-sentence technical mini-report about one result: state one direct observation, one possible explanation, one strong evidence-based deduction and one procedural requirement. Use a different specialised verb in each sentence.
Every core modal is followed by the correct verb form.
The strength of each claim matches the evidence.
Negation does not accidentally change possibility into prohibition.
Specialised verbs identify the operation precisely.
03 · Worked examples
Observe form, function and meaning together
The revised model may reduce the calibration bias.
Mô hình sửa đổi có thể làm giảm sai lệch hiệu chỉnh.
The tide may be rising faster than expected.
Mực triều có thể đang tăng nhanh hơn dự kiến.
The estimate might have been affected by sensor drift.
Ước lượng có thể đã bị ảnh hưởng bởi độ trôi cảm biến.
Can the method reproduce the observed trend?
Phương pháp có thể tái hiện xu thế quan trắc không?
04 · High-risk contrast
Explain why one form fails, not only which answer is correct
The model can to reproduce the peak.
The model can reproduce the peak.
A core modal is followed directly by the base form; to belongs to semi-modal or infinitival structures, not to the core-modal complement.
05 · Mastery check
Apply the rule before marking the lesson complete
Which sentence is grammatically acceptable in the target system?
Which description best defines “modal auxiliary”?
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 modal architecture to control claim strength in Writing, express reasoned possibilities in Speaking and avoid form errors such as modal + to, modal + V-s or broken perfect/passive chains. A precise modal plus a specialised verb such as indicate, verify, mitigate or implement is normally stronger than an impressive-looking but vague auxiliary stack.
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.