Modal verbs are used to express ideas such as ability, permission, obligation, necessity, suggestion and intention. The modal verbs include can, could, may, might, shall, should, will, would, must, and ought to. They have certain grammatical characteristics - they are followed by an infinitive without "to", they don't have past or present forms, and they don't take "s" in the third person singular. Modal verbs like can, may, must express possibility and certainty, while should and ought to provide suggestions and indicate what is advisable. Modal verbs allow speakers to express obligations, probabilities, intentions and degrees of certainty.