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Must/mustn’t
Use must + infinitive without to
Must is a modal verb, and modal verbs are followed by an infinitive without to.
No -s
Modal verbs don’t add -s after he/she/it.
Use must/mustn’t in negatives and questions
Must is an auxiliary verb. This means that it has a negative and an interrogative form. Do not use do/did in negatives and questions.
Have to/don’t have to
Use do and did in negatives and questions
The verb have in have to is not an auxiliary verb. Use do/don’t, does/doesn’t, did/didn’t in questions and negative sentences in present and past.
No contraction
The verb have in have to cannot be contracted to ‘ve.
Past and future forms
Only have to has a past and a future form. Must does not have a past or a future form.
Must vs have to / mustn’t vs don’t have to – Use
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Have to
Rules and obligations
We use have to 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.
Must
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.
Don’t have to
Don’t need to – not necessary
If you don’t have to do something, it means that you ‘don’t need to’ do something (there is no obligation). You can do it but you don’t need to do it if you don’t want to do it.
- 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).
Mustn’t
Prohibition
Use mustn’t to talk about something that you can’t do. It’s necessary that you don’t do it.
- You mustn’t call me before 8. (=You can’t call me before 8.)
- I mustn’t lose my concentration now.
Must vs have to
Must
Obligation from the speaker
We normally use must to talk about obligations that come from the opinion of the speaker. The speaker thinks it’s necessary or important to do something.
- MANAGER: You must get up early tomorrow. The meeting is at nine. (=The obligation comes from the manager. The manager thinks it’s necessary.)
- DOCTOR: You must stop smoking. (=It’s the doctor’s opinion. The doctor thinks it’s necessary.)
- SMOKER: I must stop smoking. (=It’s the smoker’s opinion. The smoker thinks it’s necessary to stop smoking.)
Have to
External obligation
We normally use have to when there’s an external obligation. The obligation doesn’t come from the speaker’s opinion; another person thinks it’s necessary.
- WORKER: We have to get up early tomorrow. The meeting is at nine. (=The obligation comes from the manager, not from the worker.)
- SMOKER: I have to stop smoking. (=It’s the doctor’s opinion, not the smoker’s opinion. The doctor thinks it’s necessary.)
Mustn’t vs don’t have to
Don’t have to
You don’t need to do it; 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
Prohibition; it’s necessary that you don’t do it
We say that we mustn’t do something when we cannot do something; it is necessary that we don’t do something.
- You mustn’t wait here. (=You cannot do it; it’s against the rules)