Modal Verbs of Obligation
Must
We use must to express a strong obligation, duty, or commitment. When we use must, the obligation is based on the opinion of the speaker.
- You must work harder.
- I must get a haircut.
- She must get here before 9 AM.
Must is also common in formal writing regarding rules or laws. You will see must used by organizations and governments, and it is usually in the passive form.
- Books must be returned by 6 PM. (library)
- English must be spoken inside the school at all times. (school)
- Cell phones must be put in airplane mode while you are flying. (government)
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