Modal verbs are helping verbs that are used with the base form of main verbs to express meanings like ability, permission, obligation, possibility, and more. Some common modal verbs are can, could, may, might, must, shall, should, will, would. Modal verbs are followed by the infinitive form of a main verb without using "to." They do not have infinitive or -ing forms themselves. Examples of proper modal verb usage include "Mary could play the piano" and "They can't be serious."