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modal auxiliary verb
A modal auxiliary verb, often simply called a modal verb or even just a modal, is used to change the meaning of other verbs (commonly known as main verbs) by expressing modality—that is, asserting (or denying) possibility, likelihood, ability, permission, obligation, or future intention.
Modal verbs are defined by their inability to conjugate for tense and the third person singular (i.e., they do not take an “-s” at the end when he, she, or it is the subject), and they cannot form infinitives, past participles, or present participles. All modal auxiliary verbs are followed by a main verb in its base form (the infinitive without to); they can never be followed by other modal verbs, lone auxiliary verbs, or nouns.
1. Philosophy logic
a. qualifying or expressing a qualification of the truth of some statement, for example, as necessary or contingent
b. relating to analogous qualifications such as that of rules as obligatory or permissive
2. Metaphysics of or relating to the form of a thing as opposed to its attributes, substance, etc.
(Of an interface) Having modes. Modeless interfaces are generally considered to be superior because the user does not have to remember which mode he is in.
See modal logic.
In MS Windows programming, A window with the label "WS_MODAL" will stay on the screen and claim all the user-input. Other windows can only be accessed if the MODAL window is closed. Such a window would typically be used for an error dialog box to warn the user for something important, like "Critical error, shut down the system and restart".