Must, Have to, Should: Obligation in English

Obligation in English uses a small set of modal verbs and modal-like structures. Must and have to say something is necessary; should gives advice or a weaker obligation. The two real traps are the negatives — mustn't and don't have to mean opposite things — so we'll spend most time there.

Must vs have to

Both mean "it's necessary". The difference is small: must often feels like the speaker's own feeling or a rule; have to points to an outside rule or fact. In everyday speech they overlap.

Note: must has no past form. For the past, use had to: "I had to work late yesterday." For explicit future obligation, will have to is common: "I'll have to work late tomorrow." Must can also refer to future rules or instructions: "You must submit it tomorrow."

Unlike must, have to changes with the subject and tense: she has to, I had to, we'll have to.

The big one: mustn't vs don't have to

mustn't = it's prohibited, don't do it. don't have to = it's not necessary, but you can if you want. Opposite meanings.

Should — advice or weak obligation

Use should and shouldn't for advice, recommendations, and weaker obligations — softer than must or have to.

Common mistakes

Using mustn't to mean "not necessary".

Adding -s or "to" to a modal. Must and should never change and take the base verb. Have to is different: it changes like a normal verb — she has to, I don't have to, did you have to?

Using must in the past. "I must work late yesterday" → I had to work late yesterday.

Quick exercise

Choose must, have to, mustn't, don't have to, or should.

1. You ___ touch that switch — it's dangerous.
2. It's free entry, so you ___ pay.
3. I ___ get up early tomorrow; I have a flight.
4. You look tired. You ___ get some rest.
5. Drivers ___ stop at a red light.