Semimodals

Semimodals are a subclass of auxiliary verbs that express modality but are distinct from the core modals in a language. They typically combine with nonfinite complements, such as to-infinitives, and often behave syntactically like auxiliary verbs rather than lexical verbs. In English, common semimodals include be able to, have to, need to, ought to, and dare in certain uses, as well as the periphrastic be to and used to.

Semimodals cover a range of modal meanings. Be able to expresses ability or potential, have to and must convey obligation or necessity, ought to signals advisability or moral obligation, and used to marks past habituality or state. The exact interpretation can vary by context and language. Some items, like be able to, are used to extend the temporal and aspectual scope of modality (for example, was able to in the past), whereas simple modals like can or must have more fixed morphosyntactic properties.

Syntactically, semimodals share many properties with modals in that they do not inflect for person or number and they attach to the main verb through a periphrastic construction. They may require a following to-infinitive or a bare infinitive, and their tense or aspect is often expressed through the accompanying auxiliary or the infinitive form rather than through the semimodal itself. This contrasts with lexical verbs that carry tense and aspect more directly.

Across languages, the presence and behavior of semimodals vary. Some languages have dedicated semi-modal verbs with clear modal semantics, while others use periphrastic constructions to achieve similar meanings. The label semimodal is primarily a descriptive term used in grammars and typological studies, with its exact membership differing by analysis.