Choose the correct forms, don’t have to, mustn’t, to complete the sentences below.
have to, don’t have to, must, mustn’t – table
have to/don’t have to
Rules and obligations
We use have to talk about rules and obligations, something that is necessary.
- I have to get to work before 7 tomorrow.
- The car broke down and we had to call a taxi.
do and did for the negative and interrogative forms
The verb have in have to is not an auxiliary verb. We need do/don’t does/doesn’t, did/didn’t to make questions and negative sentences.
- Do you have to get up early tomorrow? (NOT
Have you to get up early…)
- Did you have to buy a new sound card?
- We don’t have to stay until the end.
The verb have in have to cannot be contracted to ‘ve.
- I have to study for the exam tomorrow. (NOT
I’ve to study…)
Don’t need to
If you don’t have to do something, it means that you ‘don’t need to’ do something (there is no obligation).
- You don’t have to pick me up at the station. I can take a taxi. (=You can pick me up, but you don’t need to do it).
Rules and obligations
Must is very similar to have to. We use must + infinitive (without to) to talk about rules and obligations.
- I must go to the doctor.
- I must get up early tomorrow.
We use mustn’t to talk about something that you can’t do (prohibition).
- You mustn’t call me before 8. (=You can’t call me before 8)
- I mustn’t lose my concentration now.
Difference between have to and must
External obligation or obligation imposed by the speaker
We normally use have to when there’s an external obligation, and must when the obligation comes from the opinion of the speaker.
- We have to get up early tomorrow. The meeting is at nine. (External obligation)
- We must get up early tomorrow if we want to finish painting. (It’s the speaker’s opinion)
- You must be more patient. (It’s the speaker’s opinion)
Past and future forms
Only have to can have a past and future form. Must does not have a past or future form.
- I had to take a taxi because it was raining.
- You will have to take a taxi if it rains tomorrow. (NOT
Difference between don’t have to and mustn’t
don’t have to= not necessary; no obligation
Don’t have to and mustn’t have opposite meanings. We say that we don’t have to do something when we don’t need to do something; we can do it, but it’s not necessary.
- You don’t have to wait here. (=You can do it, but it’s not necessary)
mustn’t = necessary; prohibition
We say that we mustn’t do something when we cannot do something; it is necessary that we don’t do something (prohibition).
- You mustn’t wait here. (=You can’t do it; it’s against the rules)