Modal verbs are verbs that go before other verbs to add nuance or meaning. Some common modal verbs are can, must, could, have to, might, will, would, and should. Each modal verb conveys a slightly different meaning, such as ability (can/could), possibility (can/could/may/might), obligation (have to/must/should), permission (may/could/can), or certainty (must). Modal verbs are invariant and do not require auxiliary verbs like do/does/did. They change form in negatives and questions.