modalauxiliary

Modal auxiliary, or modal auxiliary verb, is a class of auxiliary verbs used to express modality—possibility, necessity, obligation, permission, or ability. In English, core modals include can, could, may, might, shall, should, will, would, and must. Some analyses also include need and dare as marginal modals. Modals combine with a main verb in its base form (bare infinitive): She can swim, They should study. They do not take “to” before the infinitive, and they do not inflect for tense or agreement in the same way as main verbs, making them defective in that sense.

Syntactically, modals are auxiliary verbs that appear without do-support in negation or question formation: You cannot go; Can you swim? Contractions are common (can't, won't, shouldn't, wouldn't). The forms for tense and aspect are expressed with special periphrastic structures, rather than through modal inflection. For example, the past of can is could, the past of will is would, and must has no true past form; past necessity is usually expressed as had to or must have for deduction.

Semantically, modals express different modalities. Deontic modality concerns permission, obligation, or prohibition (You must finish your work; You may leave). Epistemic modality conveys the speaker’s evaluation of possibility or deduction (She must be tired; It might rain). Some languages distinguish these categories more clearly than others, and many use additional devices to express related notions.

In summary, modal auxiliaries are a small, distinct class of auxiliary verbs that modify the main verb to convey likelihood, obligation, capability, or permission, without full tense or agreement inflection.