Must is a modal verb.
Structure and forms:
(pro)noun + must + verb
Affirmative: You must close the window.
Negative: You mustn’t close the window.
Interrogative: Must you close the window?
Short answers: Yes, you must. Or, No you mustn’t.
1. Obligation or necessity: I must get up early to finish my homework.
(The speaker thinks it is necessary. (It is an internal obligation. We use “have to” for extarnal obligation. ))
2. Certainty or strong probability: He must be over eighty, he was born in 1930.
(The speaker makes a deduction using the available information. )
- You must do your homework.
- You must be more polite.
- He must choose one.
- The bags must be heavy.
- You mustn’t speak loudly!
- You must speak English!