Modals of obligation & advice: must / have to / should
A2 26 / 50
In short
Use must and have to to say something is necessary, and should / ought to to give advice. Be careful: mustn’t means “don’t do it - it’s forbidden”, but don’t have to means “you can if you want - it’s not necessary”.
The rule
These verbs all come before the base form of another verb (no to, except have to, ought to and need to).
Obligation - something is necessary:
- must = strong obligation or a rule, often the speaker’s own feeling: You must wear a seatbelt. / I must call her.
- have to / has to = obligation from outside (a law, your job, the situation): I have to work on Saturday.
In everyday speech the two often overlap. The big difference appears in the negative:
| Negative | Meaning | Example |
|---|---|---|
| mustn’t | It is forbidden. Don’t do it. | You mustn’t smoke here. |
| don’t / doesn’t have to | It is not necessary. It’s optional. | You don’t have to come. |
Advice - a good idea:
- should / shouldn’t = advice, a recommendation: You should rest. / You shouldn’t worry.
- ought to = the same as should, a little more formal: You ought to see a doctor.
- had better (‘d better) = strong advice or a warning about now, often with a bad result if you don’t: You’d better hurry.
Past obligation: There is no past form of must for obligation. Use had to for everything in the past: I had to leave early yesterday.
Semi-modals (similar meaning):
- need to = it is necessary: I need to buy milk. (negative: don’t need to)
- be supposed to = it is expected, that is the plan/rule: We’re supposed to wear black.
Forms & patterns
| Structure | Pattern | Example |
|---|---|---|
| must | subject + must + base | She must go now. |
| mustn’t | subject + must not / mustn’t + base | He mustn’t be late. |
| have to | subject + have/has to + base | They have to pay. |
| have to (question) | Do/Does + subject + have to + base | Do you have to leave? |
| have to (negative) | subject + don’t/doesn’t have to + base | We don’t have to pay. |
| should | subject + should(n’t) + base | You should try. |
| ought to | subject + ought to + base | You ought to ask. |
| had better | subject + ‘d better (not) + base | We’d better go. |
| had to (past) | subject + had to + base | I had to wait. |
Note: must, should and ought to never add -s in the third person, and never use do/does in questions or negatives. Only have to and need to use do/does.
Examples
| English | Note |
|---|---|
| You must wear a helmet on the bike. | Strong rule the speaker insists on. |
| I must remember to charge my phone. | A strong personal need / reminder. |
| Anna has to wear a uniform at work. | Outside obligation - it’s the company’s rule. |
| Do you have to get up early tomorrow? | Question with have to uses do. |
| Passengers mustn’t open the doors. | Prohibition - it is forbidden. |
| You don’t have to finish it today. | No obligation - it’s optional, your choice. |
| You should drink more water. | Friendly advice / recommendation. |
| You ought to apologise to her. | Advice, slightly more formal than should. |
| It’s late - we’d better leave now. | Strong advice/warning about the present. |
| Sorry, I’m late - I had to take the dog out. | Past obligation; no past form of must. |
Common mistakes
- You don’t must smoke. -> You mustn’t smoke. - must never uses do; the prohibition form is mustn’t.
- You mustn’t pay, it’s free. -> You don’t have to pay, it’s free. - “not necessary” is don’t have to, not mustn’t (forbidden).
- Does she must work today? -> Does she have to work today? - use have to (with do/does) for questions, not must.
- I must to call him. -> I must call him. - no to after must, should or mustn’t.
- Yesterday I must leave early. -> Yesterday I had to leave early. - past obligation is always had to.
- You should to rest. -> You should rest. - should takes the base form, no to.
Tips
- Memory hook: mustn’t = stop! (forbidden), don’t have to = relax (it’s optional). These two are NOT the same.
- For your own strong feelings or written rules, must fits best; for everyday outside obligations and questions, have to sounds more natural.
- had better is stronger than should - it hints at a bad result: You’d better save your work (or you’ll lose it).
- There is only one past for obligation: had to. Don’t look for a past of must.