Modal verbs are used to express ideas like ability, permission, possibility, obligation, and logical deductions. Some common modal verbs are can, could, should, may, might, will, would, must. Modal verbs don't take 's' in the third person singular and are always followed by a bare infinitive without 'to'. They are also used in questions without helping verbs. Modal verbs have different meanings depending on context, such as expressing ability, permission, possibility, obligation, requests, and logical assumptions.