What is the difference between must and have to?

In the present they often mean the same thing: a required action. Must tends to come from the speaker (You must sign the harvest rota), while have to tends to come from an outside rule or circumstance (Plot-holders have to pay the levy). Have to also supplies the past and future (had to, will have to), which must cannot do for obligation.

What is the difference between mustn't and don't have to?

Mustn't means the action is forbidden: You mustn't climb the cider press. Don't have to means the action is not required: You don't have to bring a crate. One is a ban; the other is a choice. Needn't and don't need to belong with don't have to, not with mustn't.

How do you talk about obligation in the past?

Use had to for a past duty and didn't have to for a duty that did not exist: We had to move the hives before the gale. They didn't have to water that week. Must have plus a participle is a deduction about the past (They must have locked the shed), not a remembered rule.

When do I use should instead of must?

Use should (or ought to) when you are advising, not requiring. You should label the crates is a recommendation; You must label the crates is a rule. In questions, should I asks for advice: Should I rinse the press tonight? Must I asks whether the action is compulsory.

Is have got to the same as have to?

In the present, yes, especially in spoken British English: We've got to finish before the frost means We have to finish before the frost. Have got to is unusual in the past and in careful writing; had to and will have to take over there. The negative is haven't got to or, more often, don't have to.

English grammar · Intermediate

Modals of Obligation: Must, Have To, Should, and Need To

Written by Eleanor Whitfield, Lead English Curriculum Editor · Reviewed September 2026

Modals of obligation tell you how necessary an action is: a rule (must, have to, have got to), advice (should, ought to), or a practical need (need to). The negative side splits in two, and that split is the main trap: mustn't forbids an action, while don't have to and needn't say it is not required. Have to is not a true modal, but it does the same job as must and supplies the missing past and future forms.

Volunteers must sign the harvest rota before they use the cider press.

must plus a bare verb: a firm rule of the orchard

What it is

Obligation is a scale, not a single word. At the top sit must and have to: the action is required by a rule, a law, or a circumstance. In the middle sit should and ought to: the action is the better choice, but nobody will fine you for skipping it. Need to sits with practical necessity rather than a written rule (You need to water the saplings before dusk). English also marks the opposite of obligation in two different ways. Mustn't means the action is banned. Don't have to and needn't mean you are free not to do it. Mixing those two negatives is the error this page exists to stop.

You must wear a high-vis vest in the orchard. (a site rule) You should label the crates. (good practice, not a bylaw) You don't have to stay for pressing. (optional, not forbidden)

How to spot it

Look for must, should, ought to, or need followed by a bare verb, or for have to / have got to / need to followed by a to-infinitive. If the sentence names a rule, a duty, advice, or a ban, you are in this set. If it asks whether an action is allowed (Can I use the press?), you have crossed into permission.

Plot-holders have to lock the compost tumbler. (have to: an external rule of the site)
You shouldn't leave windfalls in the grass. (shouldn't: advice, not a ban)
Visitors mustn't climb the cider press. (mustn't: the action is forbidden)
You don't have to bring your own crate. (don't have to: optional, not banned)

Have got to is a spoken British equivalent of have to (We've got to finish before the frost). Ought to means the same as should and is slightly more formal. Need as a true modal (Need I sign?) is rare outside careful British English; need to is the everyday form.

Modals of Obligation Quick Reference

Obligation, advice, and the two negatives

Form	Force	Example
must + bare verb	strong obligation, often the speaker's rule	You must sign the harvest rota.
have to / have got to + verb	obligation from outside: a law, a job, a timetable	We have to close the orchard at dusk.
should / ought to + bare verb	advice or the expected choice	You should rinse the press after each batch.
need to + verb	practical necessity, not a written rule	You need to water the new saplings.
mustn't + bare verb	prohibition: do not do this	You mustn't leave the electric fence off.
don't have to / needn't + verb	no obligation: the action is optional	You don't have to stay for pressing.
had to + verb	past obligation (must has no past form of its own)	We had to move the hives before the storm.

Common Modals of Obligation Examples in English

Grouped by how strong the speaker's push is, because the grammar of each form follows that strength:

Rules and Duties

All volunteers must sign in at the shed.: must: a rule the speaker treats as fixed
Plot-holders have to pay the water levy each March.: have to: a council requirement
We've got to empty the press before the frost.: have got to: spoken, same force as have to
Stewards need to check the electric fence twice a day.: need to: a practical duty of the job

Must often comes from the speaker (a steward laying down a rule). Have to often comes from outside (a levy, a timetable, the weather).

Advice and Expected Choices

You should label every crate with the plot number.: should: the better practice
People ought to wear gloves when they handle the press.: ought to: same as should, a little more formal
You shouldn't stack wet wood against the shed.: shouldn't: a warning, not a ban
New volunteers should watch one pressing before they run the mill.: should for a recommended first step

Advice can be ignored without breaking a rule. That is the whole difference between should and must.

Prohibition vs. No Obligation

You mustn't climb on the cider press.: mustn't: climbing is banned
Children mustn't go near the electric fence.: mustn't: a safety ban
You don't have to bring your own crate.: don't have to: optional, the site has spares
You needn't stay after the last bag is filled.: needn't: British, same as don't have to

Mustn't and don't have to look like a pair. They are opposites. One forbids; the other frees you.

Past and Future Obligation

We had to move the hives before the gale.: had to: must has no past of its own
They didn't have to water that week; the rain did it.: didn't have to: past lack of obligation
You'll have to book a slot for the press in October.: will have to: future obligation
I shouldn't have left the tumbler unlocked.: shouldn't have: regret about a completed action

For past and future, English borrows have to. Must stays in the present (and in deduction, which is a different page).

How Modals of Obligation Work

True modals share one grammar; have to and need to follow ordinary-verb rules, which is why they can do what must cannot.

Must, Should, and Ought To Take a Bare Verb

Must and should are true modals: no -s, no to, no do. You must signs and you must to sign are both wrong. Ought is a modal in meaning but keeps to: you ought to rinse the press. Questions and negatives sit on the modal itself: Must we sign in? You shouldn't stack wet wood.

Must we lock the shed? You shouldn't leave the fence off.

the modal inverts for the question and takes n't for the negative

Must and should: no to, no do.

Have To Inflects Like an Ordinary Verb

Have to is a semi-modal. It takes -s (she has to), a past (had to), a future (will have to), and do-support in questions and negatives (Do we have to sign in? You don't have to stay). That flexibility is why English uses it wherever must has no form of its own. Have got to is a spoken present-tense variant; it does not normally appear in the past.

Does she have to pay the levy? She had to last year.

do-support in the question; had to for the past

Have to bends; must does not.

Mustn't Bans; Don't Have To Frees

The negative of must is not a milder must. Must not puts a stop on the action: it is against a rule or unsafe. Don't have to and needn't take the requirement away and leave a choice. You must not climb the press means stay off it. You don't have to climb the press means nobody is requiring it, and climbing remains possible. Needn't is the British modal negative of need; don't need to is the everyday equivalent.

You mustn't leave the fence off. You don't have to stay for pressing.

first sentence a ban, second sentence a choice

Mustn't = banned. Don't have to = optional.

Should Is Advice, Not a Rule

Should and ought to sit one step down from must. They name the better or expected action without making it compulsory. In a question, should I asks for a recommendation (Should I label the crates tonight?). Should have plus a past participle looks back with regret or criticism: I should have locked the tumbler. That past comment belongs with past modals; the present form stays on this page.

You should rinse the press. I should have rinsed it yesterday.

present advice vs. regret about a finished action

Should advises; must requires.

Need To Is Practical, Need Is Rare

Need to behaves like have to: it inflects (needs to, needed to) and takes do (Do we need to water them?). The true modal need (Need I sign? You need not wait) survives in careful British English and on notices. In speech, don't need to and needn't both mean the action is unnecessary. Needn't have plus a participle means you did it, but it was unnecessary: You needn't have brought extra crates; we had plenty.

You need to water the saplings. You needn't bring extra crates.

need to for a practical job; needn't for something unnecessary

Need to for a job; needn't for no job.

Common Mistakes with Modals of Obligation

Incorrect: You mustn't to lock the shed.

Correct: You must lock the shed.

Must behaves like can and should: the following verb stays in the dictionary form with no to. Keep to for have to and need to, which are not core modals.

Incorrect: You mustn't bring a crate if you don't want to.

Correct: You don't have to bring a crate if you don't want to.

Must not marks a ban. The speaker here is lifting a requirement, so the crate is optional: don't have to or needn't. Mixing the two negatives is the classic obligation mix-up.

Incorrect: Yesterday we must move the hives.

Correct: Yesterday we had to move the hives.

Must does not have a past form for obligation. A finished duty takes had to. Must have moved would be a guess about the past, not a report of a duty.

Incorrect: Does she must pay the levy?

Correct: Does she have to pay the levy? / Must she pay the levy?

Must already builds its own question (Must she pay?). It cannot take do. If you want do-support, switch to have to, which is an ordinary verb in this use.

Had To, Should Have, and Need Not

Obligation looks different once you leave the present, and a few older forms still appear on signs.

Past obligation is had to

Must has no past-tense form for obligation. Use had to for a duty that existed then, and didn't have to for a duty that did not.

We had to drain the press before the freeze.: a past duty, completed
They didn't have to water; it rained all week.: no past duty arose
Did you have to wear the vest on Sundays too?: question about a past rule

Must have drained would mean a deduction (the evidence says they drained it), not a remembered duty.

Should have for a missed better action

Should have plus a past participle comments on a completed action: it was the better choice, and it was not taken.

I should have locked the tumbler.: regret: locking was the better action
They shouldn't have left the fence off overnight.: criticism of a completed choice
You ought to have labelled the crates.: ought to have: a slightly more formal should have

This form is shared with past modals. On this page it is the past of advice, not of deduction.

Need not on notices

Need not (or need not + verb) still appears on signs and in careful British writing for 'it is not required'. Don't need to is the spoken default.

Visitors need not book for the open afternoon.: notice style: booking is optional
You don't need to bring a crate.: the same meaning in speech
You needn't have washed the spare vats; they were clean.: the extra work happened, and it was unnecessary

Needn't have (you did it, wasted effort) is not the same as didn't need to (you may or may not have done it; it wasn't required).

Quick Check: Modals of Obligation

Choose the form that matches the force of the sentence: must, have to, should, mustn't, or don't have to.

1. Plot-holders ___ pay the water levy each March. (council rule)

Answer: have to

external requirement, so have to

2. You ___ climb on the cider press. (banned)

Answer: mustn't

prohibition, not a lack of duty

3. You ___ bring your own crate; the shed has spares. (optional)

Answer: don't have to / needn't

no obligation, the action remains allowed

4. Last October we ___ empty the press before the frost. (past duty)

Answer: had to

must has no past of obligation

5. New volunteers ___ watch one pressing first. (good idea, not a bylaw)

Answer: should / ought to

advice rather than a rule

Modals of Obligation FAQs

What is the difference between must and have to?: In the present they often mean the same thing: a required action. Must tends to come from the speaker (You must sign the harvest rota), while have to tends to come from an outside rule or circumstance (Plot-holders have to pay the levy). Have to also supplies the past and future (had to, will have to), which must cannot do for obligation.
What is the difference between mustn't and don't have to?: Mustn't means the action is forbidden: You mustn't climb the cider press. Don't have to means the action is not required: You don't have to bring a crate. One is a ban; the other is a choice. Needn't and don't need to belong with don't have to, not with mustn't.
How do you talk about obligation in the past?: Use had to for a past duty and didn't have to for a duty that did not exist: We had to move the hives before the gale. They didn't have to water that week. Must have plus a participle is a deduction about the past (They must have locked the shed), not a remembered rule.
When do I use should instead of must?: Use should (or ought to) when you are advising, not requiring. You should label the crates is a recommendation; You must label the crates is a rule. In questions, should I asks for advice: Should I rinse the press tonight? Must I asks whether the action is compulsory.
Is have got to the same as have to?: In the present, yes, especially in spoken British English: We've got to finish before the frost means We have to finish before the frost. Have got to is unusual in the past and in careful writing; had to and will have to take over there. The negative is haven't got to or, more often, don't have to.