We use must in affirmative sentences to say what is necessary and to give strong advice and orders to ourselves or other people.
- Children must learn good habits. (Necessity)
- Our companies must increase their productivity. (Necessity)
- You really must stop smoking. (Necessity)
- That child must learn to say ‘thank-you’.
Talking about obligation
Must can be used to talk about obligation. It is stronger than should and ought.
- He must take care of his children.
- She must mend her ways.
- You must apologize to her.
- They must find a solution to the problem.
- He must find a job.
Should, ought and must
Should and ought have very similar meanings and can often replace each other. Note that ought is followed by the infinitive with to.
- You should tell the truth. OR You ought to tell the truth.
- He should be punctual. OR He ought to be punctual.
- He should stop smoking. OR He ought to stop smoking.
- She should find a better job. OR She ought to find a better job.
- He should consult a doctor. OR He ought to consult a doctor.
Difference between should, ought and must
Must is stronger than should and ought. It is more like an order. Should and ought to, on the other hand, are more like pieces of advice.
- He must give up smoking. (=It is an order which is likely to be obeyed.)
- He should / ought to give up smoking. (= This is more or less a piece of advice which may or may not be obeyed.)
- He must wait. (It is an order.)
- He should wait. (It is a request or a piece of advice.)
- She must tell the truth. (It is absolutely necessary.)
- You should tell the truth. (A piece of advice)
- You must work hard.
- They must send the payment.
- I must find a better job.
- He must start coming on time.
- I must talk to him.
- You must write to her.