Virtuous English Academy logoVirtuous English Academy
A security desk with a no-entry sign, a warning triangle, a passport control gate and a shield A2 · Elementary

Must, Have To and Mustn't: Obligation (A2)

Must and have to express obligation, while mustn't expresses prohibition. This lesson explains the small but important differences between them and when to use each.

Must and have to both express obligation, and in positive sentences they are close enough that either usually works. The traditional distinction is about where the obligation comes from: must for rules the speaker imposes or feels personally, have to for external rules imposed by someone else.

The negatives, however, are not similar at all — they are opposites, and this is the single most important point in the lesson. Mustn't means it is forbidden. Don't have to means it is not necessary. You mustn't park here is a prohibition; you don't have to park here simply means you have other options.

There is also a structural difference. Must is a modal, so it never changes form and has no past or future. Have to is an ordinary verb, so it conjugates fully — had to, will have to, has to — which makes it the form you need whenever you leave the present tense.

Two scales drawn as bars that narrow as the meaning weakens. Obligation runs from must and have to, through should, could, down to do not have to. Certainty runs from must, through will and should, may might and could, down to cannot.
The strip at the bottom of the first scale is this page in a single line. Mustn’t is not the weak end of must; it is a ban, while don’t have to is permission to please yourself. They look like neighbours in a list and sit at opposite ends of the scale, and most of the exercises below are built on that one gap.

How to form Obligation: Must, Have To and Mustn't

Must has no past or future; have to supplies them. The negatives mean opposite things.
MeaningPresentPastFuture
Obligationmust / have tohad towill have to
Obligation (he/she)must / has tohad towill have to
Prohibitionmustn'tcouldn't / wasn't allowed towon't be allowed to
No obligationdon't have todidn't have towon't have to
QuestionDo you have to…?Did you have to…?Will you have to…?
Strong deductionmust bemust have been—

Obligation: Must, Have To and Mustn't: the key rules

  1. Must = obligation the speaker feels; have to = obligation from outside rules.

    I must call my mum. Staff have to wear a uniform.

  2. Mustn't = prohibition (don't do it!); don't have to = no obligation (it's optional).

    You mustn't smoke here. You don't have to come — it's optional.

  3. Must has no past or future — have to covers them: had to, will have to.

    We had to wait two hours. You'll have to book early.

  4. Questions normally use have to.

    Do I have to sign anything?

When to use Obligation: Must, Have To and Mustn't

mustn't vs. don't have to

These look like they should be similar. They are opposites, and confusing them can cause genuine misunderstanding.

mustn't — it is forbidden

You are not allowed to do it. There is a rule against it.

You mustn't smoke in here. Children mustn't play near the road.

don't have to — it is optional

It is not necessary, but you may do it if you want. No prohibition at all.

You don't have to smoke — nobody is forcing you. You don't have to come early.

Obligation: Must, Have To and Mustn't in example sentences

Obligation: Must, Have To and Mustn't in a real conversation

Pay attention to the two negatives — one forbids, the other releases.

Supervisor:You have to wear a helmet in this area.

Visitor:Do I have to sign in as well?

Supervisor:Yes. But you don't have to wear boots — you're not going on site.

Visitor:Is there anything else?

Supervisor:You mustn't take photographs anywhere in the building.

Visitor:Understood. I had to do the same at the last site.

Common mistakes (and how to fix them)

Practise Obligation: Must, Have To and Mustn't today

The negative contrast is the priority — it is the part that changes meaning most.

  1. Write four rules from your workplace or school using have to.
  2. Write three prohibitions with mustn't and three optional things with don't have to.
  3. Rewrite four present obligations in the past with had to.
  4. Explain in your own words the difference between You mustn't go and You don't have to go.

Obligation: Must, Have To and Mustn't: frequently asked questions

What is the real difference between must and have to?

In positive sentences the difference is about the source of the obligation, and it is often quite subtle. Must suggests the speaker feels the obligation personally — I must lose weight — while have to points to an external rule — I have to wear a uniform. In practice have to is far more common in everyday speech, and must is increasingly restricted to written rules, strong advice and deduction.

Why do mustn't and don't have to mean opposite things?

Because the negative attaches to different parts of the meaning. Mustn't negates the action — the obligation is not to do it, so it is forbidden. Don't have to negates the obligation itself — there is no requirement, so you are free either way. English happens to place the negation differently in the two structures, which is why they diverge so sharply.

How do I express obligation in the past?

Use had to, because must has no past form: I had to work late yesterday. For past prohibition, use wasn't allowed to or couldn't, since mustn't has no past either: We weren't allowed to leave early. Note that must have is not a past obligation at all — it expresses a deduction about the past, as in He must have missed the train.

What does must mean when it is not about obligation?

It expresses a strong logical deduction — something you are almost certain is true based on evidence: You must be exhausted after that flight. She's not answering; she must be in a meeting. The negative of this meaning is can't rather than mustn't: He can't be at home, his car has gone. This deduction use is extremely common in everyday conversation.

Keep building your A2 English

Advertisement