Use of Must and Ought (to)

MUST AND OUGHT (TO)
1.  Must is used to express duty, obligation,
     necessity, etc.; as,
          We are very late, so we must hurry.
          (Necessity)
          You must obey the law. (Duty)
          We must do some social work.
          (obligation)
2.  Ought (to) is used
     i)  to express moral obligation or duty: We
         ought to love our neighbours.
     ii) to express desirability: We ought to work
          hard to pass the examination.
    iii) to express strong probability: With so
          much of practice we ought to win the
          match.
Note: The idea of obligation can be expressed
           with 'should' or 'must' also, but 'ought' is
           used when a feeling of moral duty is
           involved.