Modal verbs, also called modal auxiliary verbs, have only one form and they do not change form for tense or person. They have no to-infinitive form, -ing form, past form or -ed form. 

 

Modal verbs are placed first in the verb phrase (after the subject) and are followed by a verb in the base form. (I can run; he must jump; they shall sing, etc.)

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Negatives are formed by adding ‘not’ after the modal verbs (will not, should not, etc. An exception is with the modal verb ‘ought to,’ where the negative ‘not’ is generally placed between ought and to, as “he ought not to go.”) 

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The subject and the modal verb change position to form questions. (Will it be a problem?

Could Mary come tomorrow?)

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Modal verbs add meaning to the main verb in a sentence by expressing possibility, ability, permission, or obligation.