Modal verbs are used to express ideas such as ability, permission, possibility, obligation, and advice. There are single concept modals like will and might that have one meaning, double concept modals like may and must that have more than one meaning depending on context, and modals in the past like would have and could have. Modal verbs are followed by the base form of other verbs and do not conjugate or take tense changes. They provide additional meaning to the main verb of a sentence.