Must and have to – Grammar rules
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To express obligation, constraint or certainty, in the present tense, we have the choice between MUST and HAVE TO. There are some small differences between the two:
MUST is a modal, it is always in the present tense and always expresses the point of view of the speaker, to give his opinion or impose an obligation on himself or someone else. It is often found in written rules or to give instructions.
HAVE TO is used to say that someone is obliged to do something because they are obliged by law to do so, a rule or they simply have no choice. Have to does not express an opinion. Rather, it translates as ‘it is necessary that…’.
To put it simply, must is more accurate and strong than have to.
- I must wake up early tomorrow.
- I have to quit smoking.
- He has to go to school tomorrow.
- Do I have to wear this suit?
- Applications must be sent to the following address.
You can’t use must in the future or in the past, you have to use have to:
- present: I must quit my job.
- past: I had to quit my job.
- future: I will have to quit my job.
⚠️ We can not say ‘I will must…’ or ‘She had must…’, these are mistakes!!!!
MUST NOT (MUSTN’T) and DON’T HAVE TO
Remember, the negative form of Must and Have to are completely different, do not confuse them!
Must not expresses a prohibition (to forbid something), something that it is necessary not to do (you can also use mustn’t instead of must not, it’s the same thing).
Doesn’t have to / Don’t have to express an absence of obligation and means “not to be obliged to…”. Do not confuse:
- You don’t have to leave, but you can if you want to (= You needn’t leave)
- You mustn’t leave (it’s a prohibition).
- He’s rich. He doesn’t have to work.
- You’re pregnant. You mustn’t smoke.
⚠️ To give orders, is/are can also be used, especially for children:
- You’re not to do that.
⚠️ When you want to give advice, you have to use should:
- She should not eat that.
⚠️ If you want to talk about a potential obligation, use would have to:
- You would have to work if I left my job.
⚠️ You can also use needn’t instead of don’t have to.
- She doesn’t have to eat / She needn’t eat.
⚠️ We can use have got to instead of have to.
- I‘ve got to see that movie = I have to see that movie.