English Grammar

Should, ought and must

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

• We should respect our parents.
• We ought to respect our parents.
• You should stop smoking.
• You ought to stop smoking.

Must is stronger than should and ought.

• You must stop smoking. (A command.)
  
• You should/ought to stop smoking. (A piece of advice.)
  

Must expresses certainty. Should/ought to expresses probability.

• He must be home now. (It is certain.)
  
• He should be home now. (It is probable.)
  

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