The modality system also includes a class of auxiliary verbs called semi-auxiliaries (or semi-modals or quasi-modals). These idiomatic verbs and phrasal verbs express modal (and sometimes aspectual) meaning. They often (but not always) take the form of BE+PARTICIPLE+to. These semi-auxiliaries resemble auxiliaries in that they have some equivalence of meaning. Syntactically, the semi-auxiliaries are a mixed group. The semi-auxiliaries starting with be do not need do-support in negation or question formation, but most of the others do employ do-support for those syntactic processes. (See the discussion of do-support below.)
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