Final answer:
Modal verbs are auxiliary verbs that express possibility, ability, permission, necessity, or obligation. They modify the meaning of the main verb and can indicate different degrees of certainty or probability.
Step-by-step explanation:
Modal verbs are a type of auxiliary (helping) verb that express possibility, ability, permission, necessity, or obligation. They are used to modify the meaning of the main verb in a sentence. Examples of modal verbs include 'can', 'could', 'may', 'might', 'shall', 'should', 'will', 'would', 'must', and 'ought to'.
For example, in the sentence 'She can play the piano', the modal verb 'can' indicates the ability or possibility of the subject to play the piano.
Modal verbs can also be used to express different degrees of certainty or probability. For instance, in the sentence 'It might rain later', the modal verb 'might' signals a lower level of probability compared to saying 'It will rain later'.