Must is a modal verb used to express (1) strong obligation or necessity — You must register before midnight; (2) logical certainty or deduction — That must be the right answer; (3) an emphatic recommendation — You must try the local cheese. As a noun, a must means something absolutely essential or not to be missed.
What Does Must Mean?
Must comes from Old English moste, the past tense of mot meaning "may" or "be permitted to". Over the centuries the meaning evolved from expressing permission to expressing necessity — a common semantic shift among modal verbs. It is related to Old High German muozan and Gothic gamotan. The noun sense ("an essential thing") developed in the 19th century as a figurative extension of the verb.
In modern British English, must carries three distinct functions. As an obligation marker it signals internal necessity — the speaker or writer judges something to be required. As a certainty marker it expresses a logical deduction based on available evidence. As a recommendation it conveys strong personal enthusiasm. Recognising which meaning is intended in context is a key B2-level skill.
A useful rule of thumb: if you can replace must with have to and the sentence still makes sense as a rule or requirement, it is expressing obligation. If replacing it with must certainly be works better, it is expressing certainty. The noun use is always preceded by an article or possessive: an absolute must, our must-haves.
Example Sentences by Level
| Sentence | Level & Usage note |
|---|---|
| You must wear your seatbelt in the car. | A2 — obligation (rule/law) |
| She must be at home — her lights are on. | B1 — logical deduction (present) |
| You must proofread your writing before submitting it to check for errors. | B1 — obligation (strong advice) |
| Delegates must have submitted their abstracts by the deadline to be considered. | B2 — formal written obligation |
| The discrepancy in the accounts must have arisen from an error during the migration. | C1 — deduction about the past (must + have + past participle) |
Common Collocations
| Collocation | Example |
|---|---|
| absolute must | Booking in advance is an absolute must during peak season. |
| must-have | A good dictionary is a must-have for any language learner. |
| must-see | The exhibition is a must-see — book your tickets early. |
| must-read | Her debut novel is a must-read for fans of literary fiction. |
| must-do | Visiting the castle is a must-do when you are in Edinburgh. |
| must comply | All suppliers must comply with the new data protection rules. |
| must acknowledge | We must acknowledge the contribution of our volunteers. |
| must ensure | Managers must ensure that all staff complete the training. |
| you simply must | You simply must try the bread from that bakery — it is extraordinary. |
| must not (mustn't) | Passengers must not leave luggage unattended in the station. |
Usage Notes
Three Key Uses at a Glance
- Obligation: must = the speaker sees it as necessary. Use had to for past obligation (must has no past tense form).
- Certainty / deduction: must + bare infinitive for present deductions; must + have + past participle for past deductions.
- Strong recommendation: often used informally with you to recommend something enthusiastically: You must try this!
In formal written English — contracts, regulations, and academic writing — must is preferred over have to to indicate a strict requirement. In everyday speech, have to or need to are often more natural for obligation.
Common Mistakes
Watch Out For
You mustn't to arrive late. (modal + to-infinitive is incorrect)
You mustn't arrive late. (modal + bare infinitive)
I must go there yesterday. (must cannot refer to the past directly)
I had to go there yesterday. (use had to for past obligation)
You don't must work on Sundays. (incorrect negation)
You don't have to work on Sundays. (no obligation) / You mustn't work on Sundays. (prohibition)
She must to be the new manager. (no to after a modal)
She must be the new manager. (bare infinitive after modal)