These idioms are used to describe a good or right action or something that is to occur for certain. Should, ought to, and must are sometimes called “moral imperatives."
Note that should and ought to have virtually the same meaning. They are interchangeable.
Must is simply a stronger form of should and ought to. If someone “should" do something, it means it is a good idea for him to do it. If someone “must" do something, however, it means he has no choice: the action will occur.
Sample Usage
I should read more classical literature.
(It is a good idea to read more classical literature.)
I ought to read more classical literature.
(It is a good idea to read more classical literature.)
He should wear a seatbelt in the car.
(It is right that he wear a seatbelt.)
He must wear a seatbelt in the car.
(He has no choice, he will wear a seatbelt.)
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