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Must, have to, or should
Must is a strong obligation from the speaker, have to is one that comes from outside, and should is advice or the right thing to do.
Quick rule
A strong obligation you feel? Must. One from outside? Have to. Advice? Should. And mustn't forbids, while don't have to means optional.
A strong obligation, usually coming from the speaker: I must call my mom. Mustn't means it is not allowed.
An obligation from outside: a rule, your job, the situation. Don't have to means it is not necessary.
Advice, or what you think is right: you should rest. It is weaker than must or have to.
How to choose
| Question | must | have to | should |
|---|---|---|---|
| Is it advice you can choose to ignore? | SometimesSometimes | SometimesSometimes | Yes |
| Does the obligation come from outside (a rule, a job)? | SometimesSometimes | Yes | No |
| Does its negative mean something is forbidden? | Yes | No | No |
Why it trips people up
Must and have to both express obligation, and in the affirmative they often swap. The difference shows in the negative and the past: mustn't forbids, don't have to means optional, and the past of both is had to. Should is weaker: advice, not a requirement.
How each one works
Must
Something you decide for yourself
I must remember to call her.
A strong recommendation
You must try this cake!
Formal prohibition: must not
You must not use your phone during the exam.
Have to
A rule from outside
I have to wear a uniform at work.
The past of both: had to
We had to wait an hour.
Not necessary: don't have to
You don't have to come if you're tired.
Should
Giving advice
You should see a doctor.
The sensible thing to do
We should leave now, or we'll be late.
Asking for advice
Should I call him?
What comes next
must plus verb, no to
You must wear a seat belt.
must not plus verb: forbidden
You must not park here.
have/has to plus verb
She has to work on Sundays.
will have to: future
You'll have to wait.
do you have to...? in questions
Do you have to go?
should plus verb, no to
You should drink more water.
Almost the same
You must see this movie.
A strong recommendation of your own.
You have to see this movie.
Just as strong: here there is hardly any difference.
You should see this movie.
Softer advice.
You must not eat that.
It is not allowed.
You don't have to eat that.
You can, but it is not necessary.
You should not eat that.
It is not a good idea.
In a conversation
- A:
I have to finish this report by Friday.
- B:
You should ask Pedro for help. He's done it before.
- A:
Good idea. But I must remember the meeting at three.
- B:
Don't worry, you don't have to go. It was canceled.
Common mistakes
Wrong: You don't must pay now.
Right: You don't have to pay now.
Must doesn't take do in the negative. For not necessary, use don't have to.
Wrong: We will must leave early.
Right: We will have to leave early.
Must has no future form; after will, use have to.
Memory trick
Ask who is in charge. You, or a written rule or sign: must. A rule or a job you are telling someone about: have to. Nobody, you are just advising: should. Must and have to share one past form: had to.
It depends
- Must in written rules
- Laws, signs, and notices use must even though the obligation comes from outside: All visitors must report to reception. When you describe the rule, have to is more natural: We have to sign in.
- Have got to
- I've got to go means the same as I have to go. It is informal and especially common in British English.
FAQ
Should or must: which is stronger?
Must is stronger: it presents something as required. Should gives advice or says what is right, and you can still choose not to.
Mustn't or don't have to?
Mustn't means it is forbidden: You mustn't touch that. Don't have to means it is optional: You don't have to come.