These verbs help you talk about rules, obligations and advice. In everyday American English, have to does most of the work for obligation. Must sounds formal in conversation and is used mainly for written rules and for being sure about something. Should is for advice.
The pattern
I have to work on Saturday.
You should try the tacos.
She must be tired.
01Have to: the everyday way to say it's necessary
Use have to / has to + base verb when something is necessary. Make questions and negatives with do/does/did, like a normal verb. The past is had to.
In speech, Americans also say have got to ("I've got to go"), which is often pronounced gotta. Write "gotta" only in texts and very casual messages.
- My roommate has to be at work by 7:00 a.m.
- Do we have to bring our own towels?
- I had to call the landlord twice last week.
- Sorry, I gotta run. My bus is here.
02Must: written rules and being sure
Must appears on signs, forms, contracts and official rules. If you use it for everyday obligations in conversation, it can sound bossy or overly formal. Must has no past form for obligation: use had to.
In American conversation, must is most common when you are sure something is true based on evidence.
- All passengers must show a photo ID.
- You worked a double shift? You must be exhausted.
- Someone's at the door. That must be the pizza.
03Not necessary or not allowed?
The negatives have very different meanings. Don't have to means there's no obligation; you can if you want. Must not means it's against the rules. In conversation, Americans usually say can't for that.
| Meaning | Example |
|---|---|
| Not necessary (your choice) | You don't have to wear a tie. It's casual Friday. |
| Not allowed (formal, written) | Guests must not smoke in the rooms. |
| Not allowed (conversation) | You can't park here. It's a fire lane. |
04Should: advice and opinions
Use should / shouldn't + base verb to give advice or say what you think is the right thing to do. It's softer than have to: the other person decides.
- You should get a flu shot before winter.
- We shouldn't leave the windows open when it rains.
05Common mistakes
You don't have to smoke in the building.
You can't smoke in the building.
Don't have to means it's optional; for a rule, use can't or must not.
She have to work late.
She has to work late.
Have to changes like a normal verb: he/she/it has to.
I must to finish this report.
I have to finish this report.
Must never takes to, and have to sounds more natural in conversation.
Yesterday I must stay late.
Yesterday I had to stay late.
For past obligation, use had to.