A modal verb (also modal, modal auxiliary verb, modal auxiliary) is a type of auxiliary verb that is used to indicate modality – that is, likelihood, ability, permission, and obligation. Examples include the English verbs can/could, may/might, must, will/would, and shall/should
The primary auxiliary verbs are 'be', 'have' and 'do'. These verbs modify other verbs in a full verb phrase, e.g. 'is going', 'has gone', or 'did go'. Notes. 1. When they function as modifiers, the primary auxiliaryverbs 'is', 'do' and 'have' give information about the tense and aspect of the verb phrase.