What are modal verbs?
Modals (also called modal verbs, modal auxiliary verbs, modal auxiliaries) are special verbs which are used in questions or statements along with another verb which functions as the main verb. They are different from normal verbs like "work, play, visit..." They give additional information about the function of the main verb that follows it. They have a great variety of communicative functions.
Here are some characteristics of modal verbs:
They never change their form. You can't add "s", "ed", "ing"...
They are used to indicate modality allow speakers to express certainty, possibility, willingness, obligation, necessity, ability

List of modal verbs
Here is a list of modal verbs :-
can, could, may, might, will, would, shall, should, must
The verbs or expressions dare, ought to, had better, and need not behave like modal auxiliaries to a large extent and my be added to the above list

Use of modal verbs :-
Modal verbs are used to express functions such Permission, Ability, Obligation, Prohibition, Lack of necessity, Advice, possibility or probability.

• I can drive a car. Can you drive a car?
• I could climb a tree when I was young. Could you bring me a glass of water?
• It may rain tomorrow. May I come in?
• I shall be twenty five years next year. Shall I bring you something else?
• We should finish our work on time. Should we plan the next program?
• I will do this work for you. Will you help me?
• He would have done the work if informed. Would you lend me your bicycle please?