What is a modal verb?
A small group of auxiliary verbs, called the modal verbs (or modal auxiliary verbs, modal auxiliaries, or simply modals) are only used in combination with ordinary verbs. A modal verb changes the other verb's meaning to something different from simple fact. Modals may express permission, ability, prediction, possibility, or necessity.
The principal modal verbs are: can, could, may, might, must, ought, shall, should, will, and would.
The modal verbs are different from ordinary verbs in several ways: 1) they have no inflections at all; that is, they lack an -ing form, an -ed form, and even an -s form for the third-person singular; 2) a modal verb is always followed by the infinitive form of a verb (unless that verb has already been stated) but never follows another verb; 3) modal verbs do not follow to and are not followed by to. (Ought to, like the near-modal verb have to, is a special case.)
In their simple form, modal verbs normally refer to present or future time:
I must be nearly there by now.
I might arrive a bit later than I'd anticipated.
A trip like this can take hours more than one expects.
First Known Use of modal auxiliary
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