eng7-3.2· Unit 3: Modals, Comparison & Conditionals· ~12 min

Modal Verbs: Obligation

must vs. have to — rules and necessity.

We use 'must' and 'have to' to talk about obligation, but they feel a little different. We use 'must' for a strong, personal obligation, when the speaker feels something is very important (I must call my mother today). We use 'have to' for an obligation that comes from outside, like a rule, a law or another person (Students have to wear a uniform; it's a school rule). In the third person 'have to' changes to 'has to' (She has to get up early), and in the past both 'must' and 'have to' become 'had to' (Yesterday I had to study late). Be very careful with the negatives, because they mean different things: 'mustn't' means something is forbidden or not allowed (You mustn't smoke here), while 'don't have to' / 'doesn't have to' means it is not necessary, but you can do it if you want (It's Sunday, so we don't have to go to school). After 'must', 'have to' and 'has to' we use the bare infinitive (You must wear a seatbelt). For example: 'You don't have to bring food, but you mustn't bring your phone into the exam room.'

Key terms

obligation — Something you are required to do, because it is important or because a rule, law or person says so. In English we show obligation with 'must' and 'have to' / 'has to'.
prohibition — Something you are not allowed to do; it is forbidden. We show prohibition with 'mustn't' (for example, You mustn't smoke here).
no obligation — Something that is not necessary, so you are free to do it or not. We show this with 'don't have to' / 'doesn't have to' (for example, It's Sunday, so we don't have to go to school).
must — A modal verb for a strong personal obligation that the speaker feels is important (I must call my grandmother). It is followed by the bare infinitive and has no past form of its own.
have to — A verb phrase for an obligation that comes from outside, such as a rule or law (Students have to wear a uniform). It becomes 'has to' in the third person singular and 'had to' in the past.
bare infinitive — The base form of a verb with no 'to' and no -s or -ing ending. We use it after must, have to and has to: 'He has to go', not 'He has to goes'.
Meaning: obligation, prohibition or no obligation?
ModalMeaningExample
mustObligation (strong personal feeling)I must phone my grandmother today.
have toObligation from outside (a rule or law)Students have to wear a uniform.
mustn'tProhibition (it is forbidden)You mustn't touch the paintings.
don't have toNo obligation (not necessary, but you can)The ticket is free, so you don't have to pay.
Form: have to across persons and tenses
FormWhen to useExample
have toI / you / we / they (present)We have to pay for the bus.
has tohe / she / it (present, third person)She has to get up early.
don't have toI / you / we / they (negative, present)You don't have to bring food.
doesn't have tohe / she / it (negative, present)He doesn't have to wear a tie.
had topast, all persons (must has no past form)Yesterday I had to walk to school.
✎Choosing between prohibition (mustn't) and no obligation (don't have to)
  1. 1The sentence: In the exam room you ____ talk to other students, but you ____ bring food. Choose between 'mustn't' and 'don't have to' for each gap.
  2. 2Gap 1 - is talking forbidden or just unnecessary?: Talking to other students in an exam is not allowed at all; it is forbidden. Forbidden means prohibition, so the first gap is 'mustn't'.
  3. 3Gap 2 - is bringing food forbidden or just unnecessary?: Bringing food is not against any rule; it is simply not needed. Not necessary means no obligation, so the second gap is 'don't have to'.
  4. 4Answer: In the exam room you mustn't talk to other students, but you don't have to bring food.
✎Choosing 'has to' for he / she / it
  1. 1The sentence: My sister ____ get up early because she starts work at seven. Choose the correct form of have to.
  2. 2Find the subject: The subject is 'my sister', which is third person singular (she). For he / she / it we change 'have to' to 'has to'.
  3. 3Check the verb after it: After 'has to' we use the bare infinitive with no -s, so it is 'has to get up', not 'has to gets up'.
  4. 4Answer: My sister has to get up early because she starts work at seven.
🚫Common mistake

The most common error: 'mustn't' and 'don't have to' do NOT mean the same thing. 'mustn't' = it is forbidden (You mustn't smoke here). 'don't have to' = it is not necessary, but you may if you want (It's Sunday, so we don't have to go to school). Ask yourself: is the action forbidden, or just not needed?

🚫Common mistake

In the third person singular (he / she / it) use 'has to', not 'have to': 'She has to study tonight', 'My father has to travel a lot'. The negative is 'doesn't have to'.

⚠️Caution

After must, have to and has to use the bare infinitive: no extra 'to', no -s, no -ing. Write 'He has to finish', not 'He has to to finish', 'has to finishes' or 'has to finishing'.

💡Note

'must' has no past form of its own. For obligation in the past, both 'must' and 'have to' become 'had to' for every person: 'Yesterday the bus didn't come, so I had to walk to school.'

Rules

  1. 1Use 'must' for a strong personal obligation that the speaker feels (I must finish this today).
  2. 2Use 'have to' / 'has to' for obligation from outside, like rules and laws (We have to pay; She has to wear a uniform).
  3. 3'mustn't' = it is forbidden / not allowed; 'don't have to' / 'doesn't have to' = it is not necessary (but you can if you want).
  4. 4In the past, both 'must' and 'have to' become 'had to' (Yesterday we had to wait an hour).
  5. 5After must / have to / has to, use the bare infinitive with no -s and no -ing: 'He has to go', NOT 'He has to goes'.

Practice

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10 random questions per test

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