Modals of obligation and advice
must, have to, should, ought to (review).
Modal verbs are special auxiliary verbs that express obligation, prohibition, or advice without changing form for person or number. 'Must' expresses a strong internal obligation or a firm rule that comes from the speaker ('You must finish your homework'). 'Have to' also expresses obligation but usually refers to external rules or requirements ('Students have to wear a uniform'). The negative forms differ critically: 'mustn't' means it is forbidden — you are NOT allowed to do something ('You mustn't smoke here'), while 'don't have to' means there is no obligation — you are free to do it or not ('You don't have to come early'). For advice, 'should' and 'ought to' are near-synonyms expressing a recommendation ('You should drink more water / You ought to see a doctor'). 'Had better' is slightly stronger than 'should' and often implies a warning about a negative consequence ('You had better leave now or you'll miss the bus'). For example: 'You must wear a helmet — it is the law; you don't have to wear gloves, but you should, because it is safer.'
Key terms
| Modal | Meaning | Example |
|---|---|---|
| must | Strong obligation (from the speaker/writer) | You must finish your homework. |
| have to / has to | Obligation (external rule, law, or duty) | Students have to wear a uniform. |
| mustn't | Prohibition (it is forbidden) | You mustn't smoke here. |
| don't have to | No obligation (it is not necessary, but allowed) | You don't have to come early. |
| should | Advice / recommendation | You should drink more water. |
| ought to | Advice / recommendation (same as 'should') | You ought to see a doctor. |
| Form | Meaning |
|---|---|
| mustn't (must not) | It is forbidden. You are NOT allowed to do it. |
| don't have to | There is no obligation. You are free to do it or not. |
- 1Read the situation: Sentence to complete: 'You ___ cross the yellow line on the platform. It is dangerous.'
- 2Decide the meaning: The action is dangerous and the sign forbids it, so the meaning is prohibition (NOT allowed) — not obligation, not 'no obligation', and not mild advice.
- 3Match the meaning to a modal: Prohibition is expressed with 'mustn't' / 'must not'. 'Don't have to' would wrongly mean it is optional; 'should' would be too weak for a safety rule.
- 4Answer: You must not cross the yellow line on the platform. It is dangerous.
- 1Read the situation: The park is free on Sundays, so you do not need to pay. Choose the sentence with the same meaning.
- 2Decide the meaning: Paying is simply not necessary — it is neither forbidden nor obligatory. This is 'no obligation', not prohibition.
- 3Match the meaning to a modal: Use 'don't have to' for no obligation. 'Mustn't pay' would wrongly forbid paying; 'must pay' would wrongly make it obligatory; 'should pay' would wrongly give advice.
- 4Answer: You don't have to pay on Sundays.
Do not confuse 'mustn't' with 'don't have to'. 'Mustn't' means an action is forbidden (you are NOT allowed to do it), while 'don't have to' means there is no obligation (it is not necessary, but you are free to do it). For example, 'You mustn't park here' forbids parking, but 'You don't have to park here' just says parking is optional.
A modal verb is followed by the bare infinitive (the base form of the verb), never 'to'. Write 'You must finish', not 'You must to finish'; write 'You should apologise', not 'You should to apologise'. The same applies after 'had better': 'You had better rest', not 'You had better to rest'.
Modal verbs never add -s, -ed, or -ing. There is no 'musts', 'oughts', or 'must finishing' — the form stays the same for every subject. With 'have to', however, you do change for person: use 'has to' for he/she/it ('My brother has to take his medicine') and 'have to' for I/you/we/they.
'Ought to' and 'had better' are special. 'Ought' always needs 'to' before the verb ('You ought to wear a coat' — never 'ought wear' or 'ought to to wear'). 'Had better' is a fixed phrase that always uses 'had' (even about the present or future), with the bare infinitive after it; its negative is 'had better not', not 'hadn't better'.
Rules
- 1'Must' expresses strong obligation from the speaker or writer; 'have to' usually expresses an external obligation (rules, laws, duties).
- 2'Mustn't' = prohibition (it is forbidden); 'don't have to' = no obligation (it is not necessary, but allowed).
- 3'Should' and 'ought to' both express advice or recommendation; they are interchangeable in most everyday contexts.
- 4'Had better' expresses urgent advice with an implied warning — use it when there may be a negative consequence if the advice is ignored.
- 5All modals of obligation and advice are followed by the bare infinitive (base form of the verb), never 'to' — except 'ought to' and 'had better' which use 'to' and bare infinitive respectively.
Practice
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