Have to / must (obligation)

When something is necessary — a rule, a duty, a real need — English gives you two main ways to say it: have to and must.

Often they overlap, but have to is usually the safer everyday choice: I have to go and I must go can both mean it's necessary. The differences show up in the details: how you form them, who the obligation comes from, and — most importantly — what happens when you make them negative.

Quick shortcut: for everyday obligation, have to is the safe, all-purpose choice. It works in every tense and is the more common form in speech.

Have to: the everyday obligation

Have to behaves like an ordinary verb, so it changes form with the subject and the tense. Use has to for he / she / it:

• I have to wear a uniform at work.
• She has to catch an early train.

Because it's a normal verb, you make questions and negatives with do / does (present) or did (past):

• Question: Do you have to work on Saturdays?
• Negative: He doesn't have to pay — it's free.
• Past: We had to wait an hour.

This is the big advantage of have to: it has tense forms such as had to and will have to. Must does not change form for tense, though it can still refer to the future from context: I must leave tomorrow.

Must: strong obligation without tense changes

Must is a modal verb, so it's simpler in one way — the form never changes:

• It's the same for every subject: I must, she must, they must (never ❌ she musts).
• It's followed by the base form with no to: I must go (never ❌ I must to go).
• It has no past form of its own, though it can point to the present or future from context (I must call her tomorrow). To talk about a past obligation, switch to had to: Yesterday I had to stay late (never ❌ I must stayed).

Must often carries a sense that the obligation comes from the speaker — a personal feeling, or a rule the speaker is laying down:

• I must call my mother — I keep forgetting. (I feel this myself)
• You must be quiet in the library. (a strong rule)

You'll also see must in written rules, signs and instructions: Passengers must show a ticket.

Have to vs must: where the obligation comes from

The meanings overlap, but there's a soft pattern many learners find useful:

Don't worry too much about choosing the "perfect" one — in everyday speech they're often interchangeable, and have to is rarely wrong. The pattern above is a tendency, not a hard rule.

The negatives are completely different

This is the part to get right. Don't have to and mustn't do not mean the same thing — they're almost opposites:

• don't / doesn't have to = it is not necessary (you have a choice):
  • You don't have to come if you're tired. (you can if you want — no obligation either way)
• must not / mustn't = it is forbidden (you have no choice):
  • You mustn't touch that — it's dangerous. (do not do it)
  • The full form must not is common in written rules: Visitors must not feed the animals.

So:

• ✅ You don't have to shout, I can hear you. (no need to)
• ✅ You mustn't shout in here. (it's not allowed)

Mixing these up changes the meaning completely, so it's worth a second look whenever you make one of them negative.

Quick reference

Common mistakes

• ❌ I must to go now. → ✅ I must go now. (no to after must)
• ❌ She must leaves early. → ✅ She must leave early. (must + base form, never -s)
• ❌ Yesterday I must work late. → ✅ Yesterday I had to work late. (must has no past — use had to)
• ❌ Do you must come? → ✅ Do you have to come? / formal: Must you come? (Do you have to…? is the usual everyday question)
• ❌ You don't must be late. → ✅ You mustn't be late. (must makes its own negative; no do)
• Watch the meaning: you don't have to stay (you can leave) is very different from you mustn't stay (you are not allowed to).

Quick check

Choose the right form:

1. The museum is free, so you ____ (not / have to) buy a ticket.
2. It's a secret — you ____ (must / not) tell anyone.
3. Last week I ____ (must / have to) work on Sunday.
4. ____ she ____ (have to) wear a uniform at school?

Key takeaways

• Have to and must both express obligation; for everyday use, have to is the safe, all-purpose choice.
• Have to is a normal verb — it has tenses (had to, will have to) and uses do / does / did for questions and negatives.
• Must is a modal: same form for every subject, followed by the base verb with no to, and with no past — use had to for the past.
• Must often feels like an internal or speaker-imposed obligation; have to often points to an outside rule.
• The negatives differ: don't have to = not necessary (your choice); mustn't = forbidden.