Marginal modal verbs
The central modal verbs are can, could, may, might, must, shall, should, will, would. The marginal modal verbs, sometimes called semi-modal verbs, are dare, need, ought to, used to. They share the following characteristics:
Dare, need, ought to, and used to share most of the characteristics of modal verbs but are marginal for various reasons. Unlike the central modals, ought and used are followed by to and often combine with do in negative and interrogative constructions, like a full verb: especially in England, They didn't ought to say that alongside the more traditionally acceptable They oughtn't to say that
Used to also differs semantically from central modals, since it conveys aspect (habitual situation) and not modality.
- They are auxiliary verbs.
- They have no third-person -s form: She may go, They may go (contrast She goes, They go).
- They have no non-finite forms (no infinitive, -ing participle, or -ed participle), and therefore in standard English can appear only in initial position in the verb phrase, and cannot occur with each other
Dare, need, ought to, and used to share most of the characteristics of modal verbs but are marginal for various reasons. Unlike the central modals, ought and used are followed by to and often combine with do in negative and interrogative constructions, like a full verb: especially in England, They didn't ought to say that alongside the more traditionally acceptable They oughtn't to say that
Used to also differs semantically from central modals, since it conveys aspect (habitual situation) and not modality.