Rule 1: Modal Auxiliary verbs are not used as main verbs. They are only used as helping verbs.

E.g: I can lift this box.
          (Here, "Can" is Helping verb & "lift"is Main verbs)

        He should work hard.
        (Here, "Should"is Helping Verbs & "Work" is Main Verb.) 

Rule 2: Modal Auxiliary verbs are not used in V1, V2, V3, 'ing' or 's/es' form. Their form do not change with subject, person or number.

E.g: She can help you.
       (Here, "help" is V1)

Rule 3: 'Ought' and 'used' is always followed by infinitive 'to+V1'.

E.g: We ought to respect our parents.
       ("to respect" : to+V1)
     
       He used to come late.
        ("to come" : to+V1)

More Notes on Verb, Important For SSC | IBPS | RRB | Bank Exams

• Use of Modal Auxiliary Verbs (Can & Could)
• Use of Modal Auxiliary Verbs (May, Might & Must)
• Use of Shall & Will