English — Obligation: must vs have to
Both must and have to express obligation, but the source differs. Use must for the speaker's own authority or strong personal feeling; use have to (or have got to) when the obligation comes from outside — a rule, a law, another person.
How it works
Must is a modal: same form for every subject and followed by the bare infinitive — I must go, She must go. Have to changes with the subject (I have to / She has to) and needs do/does to make questions and negatives. For the past use had to; for the future use will have to, since must has no past or future form of its own.
Examples
| Form / example | Meaning and use |
|---|---|
| I must finish this tonight. | Present — the speaker's own strong resolve or authority. |
| You must wear a seatbelt. | Present — a rule stated with the speaker's authority. |
| I have to wear a uniform at work. | Present — external obligation (an outside rule), not the speaker's choice. |
| She has to leave early today. | Present, third person — note has to for he/she/it. |
| I've got to call my boss. | Present, informal — have got to means the same as have to, common in speech. |
| I had to work last weekend. | Past — had to is the only past form; must has none. |
| You will have to book in advance. | Future — will have to expresses future obligation. |
Good to know
The negatives are not opposites. must not (mustn't) means it is forbidden — You mustn't smoke here. But don't have to means there is no obligation, you are free to choose — You don't have to come. Mixing these up changes the meaning completely.