Modals or modal auxiliary verbs are the verbs that combine with another verb to indicate mood or tense. The modal auxiliaries express the manner of actions denoted in the main verb or indicate attitudes like permission, determination, will, possibility, necessity, etc. Modals do not change their forms according to the number or person of the subject. Also, they make questions by inversion; for example, ‘she can go’ becomes ‘can she go?’
Some examples of modals are shall, will, should, would, can, may, could, might, must, ought to, used to, need and dare. ‘Shall’ for first person and ‘will’ for second and third persons are used to express future; however, ‘will’ for first person and ‘shall’ for second and third persons are used to show special determination. Modals ‘can’ and ‘may’ are generally used to seek or grant permission, ‘can’ in an informal manner but ‘may’ in a more formal and polite manner. ‘Can’ or ‘could’ also talk about a skill or ability. Modals ‘must’ and ‘ought to’ are used to express duty, obligation, necessity, etc. ‘Used to’ expresses a discontinued habit or thing.