Must is used to express several different ideas.
To express compulsion
- You must do your homework before going to bed.
- You must finish this work before Monday.
- You must attend the conference.
To express duty
- We must serve our country.
- We must love our brethren.
To express obligation
- We must respect our elders.
- We must obey our parents and teachers.
To express necessity
- We must get the roof repaired before monsoon sets in.
- You must get a good job.
To give advice
- You must change your clothes before leaving for office.
- You must stop smoking.
- You must exercise regularly.
- You must avoid fatty foods.
To express some possibility or expectation
- She must be back in a short while.
- That must be interesting.
- He must be working.
To express a logical conclusion
- She must be nearly 40.
- He must be joking.
- She must be a miserable house wife.
Must refers to the present or future. In the past tense, we use had to to express similar ideas.
- I had to do the paint job again.
- I had to wait for them for two hours.
Instead of must, we can use will have to to talk about future obligation.
- You will have to reach the station by 6.30. (OR You must reach the station by 6.30.)
The structure must have + past participle can refer to the past.
- She must have reached home by now.
- It must have been past mid-night when Dad returned home.
Must not is used to prohibit.
- You must not tease the dog.
- You must not smoke.
- You must not be late for office.
- You must not waste your money.