Modal Verbs

Modal verbs are auxiliary verbs that express modality — the speaker's attitude toward the likelihood, necessity, or desirability of a state of affairs. The core English modals are can, could, may, might, must, shall, should, will, would. Semi-modals (have to, be able to, be going to, ought to, need to) share some functions but behave differently grammatically.

Grammatical Properties

Core modals share distinctive features that set them apart from lexical verbs:

• No -s in third person: She can swim (not cans)
• No infinitive: *to must is impossible
• Followed by bare infinitive: You must go
• Negative formed with not: She cannot / can't
• No co-occurrence: *She will can come is ungrammatical (semi-modals fill this gap: She will be able to come)

Epistemic vs Deontic Modality

The fundamental semantic distinction:

Type	Concerns	Example
Deontic	Permission, obligation, volition	You must finish by Friday. (obligation)
Epistemic	Probability, inference, certainty	She must be at home. (logical deduction)

The same modal verb carries both meanings, and only context disambiguates. This polysemy is the central teaching challenge:

• He can swim (ability — deontic) vs That can be dangerous (possibility — epistemic)
• You should study (advice — deontic) vs They should be here by now (expectation — epistemic)
• She must leave (obligation — deontic) vs She must be tired (certainty — epistemic)

Modals and Time Reference

Modals interact with Tense Aspect and Time in non-obvious ways:

• Could is not always past of can: Could you help me? (present, polite request)
• Would is not always past of will: I would prefer tea (present, tentative)
• Must has no past form — had to substitutes for past obligation
• Modal + perfect infinitive creates past reference: She must have left (epistemic, past); You should have told me (deontic, past criticism)

Functional Summary

Function	Modals used
Ability	can, could, be able to
Permission	can, could, may
Obligation	must, have to, should, ought to
Prohibition	must not, cannot
Advice	should, ought to, had better
Possibility	may, might, could, can
Certainty / deduction	must, will, can't
Prediction	will, shall, be going to
Request	can, could, will, would

Teaching Implications

• Teach modals by function, not in isolation — group them around communicative needs (requesting, advising, speculating)
• Make the epistemic/deontic distinction explicit from intermediate level
• Use Speech Acts and Pragmatics to show how modal choice affects politeness and directness
• Avoid the false equivalence will = future tense — it is a modal expressing prediction, volition, or willingness