Use of Ought to and Must

Ought to
• It is used to indicate moral obligation or duty:-
(1) A soldier ought to serve his motherland.
(2) We ought to serve our ailing parents.
(3) We ought to obey traffic rules.
(4) You ought to be punctual.
(5) She ought to do this work.

Note that both 'ought to' and 'should' can be used interchangeably. The former expresses moral obligation whereas the latter expresses obligation.

Must
• It is used to indicate necessity or compulsion:-
(1) You must run fast to catch the bus.
(2) You must reach school in time.
(3) You must see the doctor at once.
(4) You must hurry, or you will miss the train.
(5) You must burn midnight oil if you want to secure good marks.
(6) We must fight for liberty.
(7) You must seek forgiveness of your teacher.
(8) She must carry out my orders.
(9) You must clear the dues by the 10th of July.
(10) A servant must obey his master.

• It is used to indicate inevitability:-
(1) All living things must eat to survive.
(2) All living things must grow.
(3) Everybody must die one day.
(4) All living things must breathe.
(5) What can't be cured, must be endured.