English grammar: Modals of obligation
When we talk about obligation in English, we can use "must", "have to", "need to" and "can't".
To say something is necessary
We use "must" to talk about obligations. Often, when we use "must", the authority for the obligation comes from the person who is speaking.
"You must do your homework every night." (Because I say you must!)
"I must stop smoking!" (Because I think it's a good idea to stop.)
Remember that "must" is a modal auxiliary verb. This means that it doesn't change its ending (I must, he must, etc) and that it's followed by the infinitive without 'to'. ("You must phone me" not "You must to phone me".)
For more information on modal auxiliary verbs, see our page on ability.
We can also use "have to" to talk about rules and regulations. The authority for the obligation doesn't come from the person who is speaking. Perhaps the rule is a general law or obligation.
"In England you have to pay tax."
"We have to check everyone's ID."
"Have to" is a normal verb. Use "do" or "does" to make a question, and "don't" or "doesn't" to make a negative.
"Do you have to vote in an election?"
"He doesn't have to wear a uniform to school."
Have got to
"Have got to" is common in British English and is stronger than "have to".
"I have got to fill in this form. The deadline is tomorrow."
"She has got to study hard to pass the exam."
To make the question and negative form, use "have", "has", "haven't" and "hasn't":
"Have you got to leave early tomorrow?"
We use "need to" to talk about what is necessary.
"You need to go to the hairdresser's. Your hair is very long."
"She needs to go to the doctor. She gets headaches every day."
"Need to" is like "have to": use do / does to make questions:
"Do you need to pass an exam to get into university?"
"Does she need to get a job?"
To say something isn't an obligation
To say there is no obligation, use "don't / doesn't have to" or "don't / doesn't need to".
"You don't have to bring food on the trip."
"She doesn't have to work in the evening."
"I don't need to pay now. I can pay later."
"They don't need to speak English in their job."
Typical grammar mistake! Be careful when you use "don't have to". It does not mean the same as "mustn't" – see below.
To say something is forbidden
To say that there is an obligation not to do something, use "mustn't".
"You mustn't play here – it's dangerous!"
"He mustn't eat peanuts. He's allergic to nuts."
We can also use "can't":
"You can't go out tonight. You've got homework."
be allowed to
"We're allowed to take an hour for lunch."
"We aren't allowed to leave early."
"Are you allowed to use the internet at work?
Should is a weak obligation, and we use it to give advice.
"You should study hard so you can pass the exam."
"He should see a doctor."
The negative form is "shouldn't":
"You shouldn't smoke. It's bad for your health."
Now try the grammar test below!
- Remember: the infinitive without 'to' after modal auxiliary verbs.
- Remember: to form a question with a modal auxiliary, invert the modal with the subject. You don't need ''do' or 'does'.
- Remember: 'must' doesn't change for the third person singular.
- Remember: use 'do' or 'does' to make a question with 'have to'.
- The negative of 'have to' is 'don't have to' or 'doesn't have to'. (Not 'haven't to' or 'hasn't to'.)
- 'Need to' is similar to 'have to'.
- Use 'mustn't' to talk about things which are forbidden.
- Use 'don't have to' or 'don't need to' to say that something isn't an obligation.
Need more practice? Check out our quiz on using can and have to.
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