We often use must to express an obligation imposed by the speaker:
Must is commonly used for the following situations:
• To give a strong personal opinion: I believe people must vote at elections.
• To impose an obligation on oneself: I really must lose some weight.
To give instructions: The electricity must always be switched off before repairs are attempted.
•We use must for something that we believe to be true because of evidence:
This must be the place - it's the only restaurant in the street.
•We use must have + past participle to express a logical deduction about the past:
There was a terrific noise last night. It must have been an explosion.