Must vs Have To
Level B1 · Modals
must and have to both express obligation — 'it is necessary to do this' — but the obligation comes from a different place. must expresses the speaker's own authority, personal judgment, or an internal sense of necessity: I must call my mother (I feel I should — it matters to me). have to expresses an external rule, law, or circumstance outside the speaker's control: I have to wear a uniform at work (the company requires it, not me). In everyday spoken English, have to is far more common overall; must tends to sound more formal, urgent, or personally emphatic.
How to use each form:
- must + base verb = the speaker's own obligation or urgency: You must be careful with that knife (I'm telling you, it matters to me). We must finish this today (I feel it's essential).
- have to / has to + base verb = an external rule or circumstance: Students have to wear a badge on campus (school rule). She has to renew her passport before she travels (a legal requirement, not her personal wish).
- must has no past tense at all — for past obligation, whatever the source, English always uses had to: I had to finish the report yesterday (not I musted or I must finish yesterday).
- must doesn't naturally form everyday questions or the future the way have to does: Do I have to come? is the normal, natural question; Must I come? is grammatically correct but sounds formal or old-fashioned. For the future, use will have to: You'll have to submit the form by Friday (not a future form of must).
The most common mistake is using must for past obligation: I must go to the doctor yesterday is wrong, because must has no past form. Past obligation is always had to, no matter where the obligation came from: I had to go to the doctor yesterday. A second frequent error is defaulting to Must you...? for an ordinary, everyday question — it sounds unnaturally formal or dated: instead of Must you work this weekend?, say Do you have to work this weekend? in normal conversation.
Look for who or what is imposing the obligation. If it's the speaker's own feeling, authority, or sense of urgency, expect must: I really must call her back. If it's an external rule, law, policy, or circumstance, expect have to: The law requires you to wear a seatbelt, so you have to wear one. For past time, expect had to regardless of the source. For questions, expect do/does + have to rather than Must...? in everyday English.