Use must or have to for things that are necessary: I have to work tomorrow. Use should for advice: You should rest. Use don't have to when something isn't necessary, and mustn't when it's not allowed: You don't have to come. You mustn't smoke here.
You have to wear a helmet on the building site.
A gauge from not allowed to necessary. Have to and must are at the top: there's no choice.
| must / have to | Necessary, an obligation: I must / have to finish this today. |
|---|---|
| should / ought to | Advice, the right thing to do: You should call her. |
| don't have to | Not necessary, but you can if you want: You don't have to come. |
| mustn't | Not allowed: You mustn't tell anyone. |
Must has no past form. For the past, use had to: I had to work late yesterday.
I have to be at work by 8.
There's no choice: a rule, a job, a law or your own strong decision.
You should try the soup. It's great.
A good idea, but the other person decides.
It's Saturday. I don't have to get up early.
You can if you want, but you don't need to.
You mustn't use your phone during the exam.
It's forbidden. Don't do it.
Common expressions worth learning as they are.
In the UK, you _____ drive on the left.
How sure, how necessary, how polite.
Paste an email or an essay. Clarigram marks the mistakes you actually make, shows the rule behind each one as a picture, and builds your practice from them.
Often both work. Must tends to show the speaker's own feeling or a written rule; have to shows an obligation from outside. In the past, only had to works.
They mean the same. Ought to is less common and a little more formal: You ought to apologise.