The modal verbs are can, could, may, might, must, should, and ought to. They are used to express ability, permission, possibility, necessity, prohibition, advice, and obligation. They have specific meanings and uses depending on whether they indicate ability, permission, possibility, necessity, advice, or obligation. They also have distinct grammatical forms depending on whether they are used in affirmative, negative, interrogative, or other contexts. The modal verbs are an essential part of English and have high frequency of use.