Should, ought to, must

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Should and ought have similar meanings. They are both used to talk about obligation, and to give advice or instructions.

Must is stronger than should and ought.

Must expresses certainty. Should/ought to expresses probability.

See also

Modal Auxiliary Verbs
Can
May and Can: differences
Could
May
Might
Will
Would
Shall
Should
Should: other uses
Must
Must: uses
Must and have to: The Difference
Ought to
Need
Had better
Should, Ought and Must: The difference
Primary auxiliaries
Verbs