modal verbs

The central modal verbs are can, could, may, might, must, shall, should, will, and would. They function as auxiliaries, not as main verbs. In The witness may have misunderstood the question, misunderstood identifies the action, have forms the perfect construction, and may presents the misunderstanding as a possibility. The modal applies to the proposition expressed by the clause, not merely to the word immediately following it.

The central modals share a distinctive grammar. They do not inflect for person or number, and they have no infinitives or participles. They occur as finite auxiliaries and take bare infinitival complements. In The application must be reviewed, must is the finite modal, be is the passive auxiliary in the infinitive, and reviewed is the main verb’s past participle. A modal can therefore form part of a longer auxiliary sequence without becoming a main verb or making every verb in that sequence a modal.

Like other auxiliaries, modals participate in subject–auxiliary inversion, negation, and ellipsis. In Could the committee reconvene?, could precedes the subject. In The committee could not reconvene, not negates the clause without do-support. In The chair cannot attend, but the secretary can, the second clause leaves attend understood. The final can remains an auxiliary.

Tense and time reference. Do not assume that could and would refer to past time wherever they occur. Compare Before the injury, Laura could lift the equipment unaided with If an assistant were available tomorrow, Laura could complete the installation. The first could has past-time reference. The second occurs in a conditional construction referring to the future; its past-tense form expresses modal remoteness rather than past time. The installation depends on a hypothetical condition.

A modal perfect likewise need not refer to an event before the moment of speaking. In By Friday, the contractor will have completed the repairs, the perfect places completion before a future reference point. Distinguish the grammatical construction—modal plus perfect—from the time to which the sentence refers.

Perfect constructions. Use have, not of, in could have, should have, would have, and similar constructions. The reduced pronunciation of have does not turn the auxiliary into a preposition.

Double modals. Combinations such as might could and might should occur in regional dialects but do not belong in standard English. Standard English does not combine two central modals in the same auxiliary sequence. When revising such a construction, preserve its intended force: might be able to retains both possibility and ability. Deleting one modal alters the meaning. A sequence such as must have been examined presents no such problem: it contains one modal, a perfect auxiliary, a passive auxiliary, and the main verb.

Related constructions. The central modals are not the only verbs with modal uses. Ought is a marginal modal auxiliary, taking a to-infinitive. Need and dare have both auxiliary and main-verb uses: compare Need we postpone the hearing? with Do we need to postpone the hearing? The first construction uses auxiliary inversion; the second uses do-support.

Had better expresses a modal meaning without belonging to the nine central modals. Retain had, either in full or contracted: We had better leave or We’d better leave. We better leave is nonstandard. See also auxiliary verbs / helping verbs; bare infinitive; can / may; may / might; shall / will; should of; could of / could have; cannot / can not.