Modal verbs a modal verb

Defectiveness[edit]

In English, modals form a very distinctive class of verbs. They are auxiliary verbs as are be, do, and have, but unlike those three verbs, they are grammatically defective. For example, have → has vs. should → *shoulds and do → did vs. may → *mayed, etc. In clauses that contain two or more verbs, any modal that is present always appears leftmost in the verb catena (chain). Thus, modal verbs are always finite and, in terms of syntactic structure, the root of their containing clause. The following dependency grammar trees illustrate this point:

The verb catenae are in blue. The modal auxiliary in both trees is the root of the entire sentence. The verb that is immediately subordinate to the modal is always an infinitive. The fact that modal auxiliaries in English are necessarily finite means that within the minimal finite clause that contains them, they can never be subordinate to another verb, e.g.,

a. Sam may have done his homework. The modal auxiliary may is the root of the clause.

b. *Sam has may done his homework. Fails because the modal auxiliary may is not the root of the clause.

a. Jim will be helped. The modal auxiliary will is the root of the clause.

b. *Jim is will be helped. Fails because the modal auxiliary will is not the root of the clause.

Such limits in form (tense, etc.) and syntactic distribution of this class of verbs are motivation of the designation defective. Other constructions are frequently used for such a "missing" form in place of a modal, including "be able to" for can, "have to" for must, and "be going to" for shall and will (designating the future). It is of note that in this way, English modal auxiliaries are unlike modal verbs in other closely related languages; see below.