Simple explanation
Understanding obligation, permission and prohibition Before choosing a modal or related structure, identify the meaning you want to communicate. Obligation means that an action is necessary. Permission means that someone is free to do something. Prohibition means that an action is not allowed. There is also lack of obligation, which means an action is optional. These meanings are related, but choosing the wrong structure can completely change your message. Expressing obligation with must and have to Both must and have to can express necessity or obligation. Must often presents an obligation strongly from the speaker's point of view or gives a firm instruction: You must be careful. Have to is especially common when circumstances, practical requirements or rules make something necessary: I have to wear an ID card at work. In everyday English, however, their meanings often overlap, so the distinction should not be treated as an absolute rule. Expressing permission with can and be allowed to Can is very common for giving and asking for permission in everyday situations: You can use my phone. Can I sit here? Be allowed to also expresses permission and is particularly useful when discussing rules or official arrangements: Visitors are allowed to take photos. Because the verb be can change for person and tense, be allowed to is also useful when can does not provide the form you need. Expressing prohibition with mustn't and can't Mustn't expresses a strong prohibition or instruction not to do something: You mustn't touch that button. Can't can also mean that something is not permitted: You can't park here. In these contexts, can't refers to permission rather than ability. The situation usually makes the intended meaning clear. The crucial difference between mustn't and don't have to Mustn't and don't have to have very different meanings. Mustn't means that an action is prohibited. Don't have to means that an action is not necessary, although you may still choose to do it. You mustn't use your phone during the exam means phone use is forbidden. You don't have to bring a laptop means bringing one is optional. Asking about obligation and permission Do I have to ...? and Do we have to ...? are common ways to ask whether something is necessary. Can I ...? is a natural everyday way to ask for permission. Am I allowed to ...? or Are we allowed to ...? focuses more directly on whether a rule or situation permits an action. Talking about past obligation and permission For a completed obligation in the past, English normally uses had to: I had to work late yesterday. For past permission, was or were allowed to can be used: We were allowed to leave early. These forms let you talk about the same concepts beyond present-time rules and situations.
Grammar formula
Obligation with must
subject + must + base verbYou must wear a seat belt.
Obligation with have to
subject + have/has to + base verbShe has to start work at nine.
Lack of obligation
subject + don't/doesn't have to + base verbYou don't have to bring any food.
Permission with can
subject + can + base verbYou can use my laptop.
Permission with be allowed to
subject + be + allowed to + base verbVisitors are allowed to take photos.
Prohibition with mustn't
subject + mustn't + base verbYou mustn't open this door.
Prohibition with can't
subject + can't + base verbYou can't park here.
Question about obligation
Do/Does + subject + have to + base verb?Do we have to book in advance?
Request for permission
Can + subject + base verb?Can I sit here?
Question about permission
be + subject + allowed to + base verb?Are we allowed to take photos?
Past obligation
subject + had to + base verbI had to work late yesterday.
Past permission
subject + was/were allowed to + base verbWe were allowed to leave early.
Core rules
- 1
Use must + base verb
Use must followed directly by the base form of the verb: You must leave. Do not add to after must. The form of must does not change with the subject.
- 2
Have to changes with person and tense
Use have to with I, you, we and they, but has to with he, she and it in the present: I have to work; she has to work. Use had to for past obligation.
- 3
Use can + base verb for permission
Can can give or request permission. It is followed by the base form without to: You can stay. Can I come in?
- 4
Use be allowed to for permission
Be allowed to is formed with the appropriate form of be followed by allowed to and the base verb: She is allowed to leave; they were allowed to leave.
- 5
Use mustn't for prohibition
Mustn't + base verb means that an action is prohibited or must not happen: You mustn't enter this room.
- 6
Use can't when something is not permitted
Can't + base verb can express lack of permission: You can't park here. Context distinguishes this meaning from inability.
- 7
Use don't have to for lack of obligation
Don't or doesn't have to means that an action is not necessary. It does not mean that the action is prohibited: You don't have to come early.
- 8
Use do or does in present questions with have to
Form present-tense questions with do or does: Do you have to work? Does she have to pay? After does, use have rather than has.
Useful examples
You must show your passport at the border.
Showing your passport at the border is required.
Must presents the action as a clear obligation.
I have to finish this report before Friday.
It is necessary for me to complete the report before Friday.
Have to describes a practical requirement or necessity.
My sister has to wear a uniform at work.
Wearing a uniform is required for my sister at her workplace.
Have changes to has with a third-person singular subject.
You can use my charger if you need it.
You have permission to use my charger if necessary.
Can gives permission in an informal everyday situation.
Employees are allowed to work from home twice a week.
Employees have permission to work remotely two days each week.
Be allowed to is useful for permission connected with a policy or rule.
You mustn't share your password with anyone.
Sharing your password with other people is not permitted.
Mustn't expresses a strong prohibition.
Sorry, you can't take photos inside the museum.
Taking photographs inside the museum is not allowed.
Can't refers to lack of permission here, not lack of ability.
We don't have to go to the office tomorrow.
Going to the office tomorrow is not necessary for us.
Don't have to removes an obligation; it does not create a prohibition.
Do I have to pay for parking here?
Is paying for parking here required?
Do I have to ...? asks whether an obligation exists.
Can I borrow your headphones for a minute?
Is it okay if I use your headphones briefly?
Can I ...? is a common way to ask for permission.
Are we allowed to bring our own food?
Do the rules permit us to bring food with us?
Be allowed to focuses directly on whether an action is permitted.
You don't have to answer every question today.
It is not necessary to answer all the questions today.
The listener may answer them all, but there is no obligation to do so.
I had to take a taxi because the last bus had already left.
Taking a taxi became necessary because there were no more buses.
Had to expresses a past necessity.
We were allowed to leave early after the meeting.
We had permission to leave earlier than usual after the meeting.
Were allowed to expresses permission in a past situation.
Common mistakes
- ❌ You must to wear a helmet. ✅ You must wear a helmet. Must is followed directly by the base form. Do not use to after must.
- ❌ She have to leave at six. ✅ She has to leave at six. With a third-person singular subject in the present, use has to.
- ❌ You don't have to smoke here. ✅ You mustn't smoke here. If the intended meaning is that smoking is prohibited, don't have to is incorrect because it only means that an action is unnecessary.
- ❌ You mustn't bring any food. We already have enough. ✅ You don't have to bring any food. We already have enough. In this context, bringing food is unnecessary rather than prohibited, so don't have to expresses the intended meaning.
- ❌ Are we allowed bring our phones? ✅ Are we allowed to bring our phones? The structure is be allowed to + base verb, so to cannot be omitted.
- ❌ He can to leave early today. ✅ He can leave early today. Can is followed by the base verb without to.
- ❌ Does she has to work tomorrow? ✅ Does she have to work tomorrow? After does, use the base form have rather than has.
- ❌ Yesterday I must work late. ✅ Yesterday I had to work late. For a completed past obligation, English normally uses had to.
Learning tips
- Identify the meaning before choosing the grammar: obligation, permission, prohibition or lack of obligation.
- Remember the contrast as mustn't = prohibited and don't have to = optional. This prevents an important meaning error.
- Never put to between must or can and the main verb.
- Use Do I have to ...? as a practical everyday pattern for checking whether something is required.
- Use Can I ...? for many everyday requests for permission; use be allowed to when the existence of a rule or permission is more central.
- Use had to for completed obligations in the past rather than trying to create a past form of must.
- Practise these structures through real contexts such as workplace rules, travel, school, public spaces and house rules rather than memorising isolated forms.
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PlannedRelated grammar
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Frequently asked questions
What is the difference between must and have to?
Both can express obligation. Must often gives strong emphasis to a necessity or instruction, while have to is very common for requirements created by rules, circumstances or practical needs. Their meanings can overlap in many everyday situations, so the distinction is not absolute.
Do mustn't and don't have to mean the same thing?
No. Mustn't means an action is prohibited. Don't have to means there is no obligation to perform the action. You mustn't leave means leaving is not allowed, while You don't have to leave means staying or leaving is your choice.
Should I use can or be allowed to for permission?
Can is very common for everyday permission and requests. Be allowed to is particularly useful when discussing rules or formal permission, and it can also be adapted easily to different tenses.
Why do we say must go instead of must to go?
Must is a modal verb, and modal verbs such as must and can are followed directly by the base form of the main verb. Therefore must go and can stay are correct.
How do I talk about obligation in the past?
Use had to for a completed past obligation: I had to work late yesterday. English does not normally create a simple past form of must for this meaning.
How can I ask whether something is compulsory?
Use Do I have to ...? or Do we have to ...? For example: Do I have to book a ticket in advance?
Can can't mean that something is prohibited?
Yes. In a sentence such as You can't park here, can't means parking is not permitted. The context makes it clear that the sentence is about permission rather than physical ability.