Modal verbs: Permission, obligation, prohibition, necessity

Permission

Can, could, may, might

The most common modal verbs to talk about permission are can, could, may and might.

• ASKING FOR PERMISSION (INFORMAL): Can I sit here?
• ASKING FOR PERMISSION (FORMAL): Could I use this chair? Might/May I use this chair?
• GIVING PERMISSION (INFORMAL): You can sit here.
• GIVING PERMISSION (FORMAL): You may use this chair.
• REFUSING PERMISSION (INFORMAL): Sorry, you can’t use this chair.
• REFUSING PERMISSION (FORMAL):  Sorry, you may not use this chair.

Was/were allowed to, couldn’t

To talk about permission in the past, we can use the forms was/were allowed to or the negative form couldn’t. We don’t use the affirmative form could to talk about permission in the past.

• He wasn’t allowed to sit down during the lesson.
• He couldn’t sit down during the lesson. 
• He could receive visits.  
• He was allowed to receive visits.

Obligation

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Must / have to

Must and have to are used to express obligation. When we use must this usually means that the obligation comes from the speaker, it’s like a personal obligation, whereas have to normally means that the obligation is external.

• I must give up smoking. (I need to, I say so)
• I have to give up smoking. (I’m obliged. My doctor says so)

In informal English can also use ‘ve got to to express obligation  (⇒ See Have – auxiliary or main verb).

• I’ve got to be there before ten. 

In the past, we use had to in every instance.

• I had to give up smoking. (because I needed to, or because my doctor forced me to).

Mustn’t / don’t have to

The negative forms mustn’t and don’t have to are completely different. Mustn’t is used to express prohibition (an obligation not to do something), whereas don’t have to is used to express an absence of obligation. (⇒ See Have to, must, should – obligation, prohibition, necessity, advice)

• You mustn’t reveal where you get the information. (=you have the obligation not to do it)
• You don’t have to arrive before 7. (=you can do it, but it’s not necessary, there’s no obligation)

Necessity

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Need to, have to, don’t need to, don’t have to

We use need to/have to or don’t need to/don’t have to + infinitive to say that something is or is not necessary.

• We need to/have to confirm our reservations before Friday. 
• You don’t need to/don’t have to believe in God to be a good person. 

Don’t need to / needn’t

We can use both don’t need to or needn’t + infinitive to say that it is unnecessary to do something. However, when we are talking about a general necessity (in general, not on one specific occasion), we normally use don’t need to, and we can use both don’t need to or needn’t + infinitive when we are talking about a specific necessity (on one specific occasion).

• The doctor said I don’t need to wear glasses. (In general, all the time)
• Tell him he doesn’t need to/needn’t wash the dishes. I’ll do it later. (On one specific occasion)

Didn’t need to / needn’t have

When something was not necessary but we did it, we can use both didn’t need to + infinitive and needn’t have + past participle.

• Thanks, it’s very beautiful, but you didn’t need to buy/needn’t have bought anything. (=you did it)

However, when something was not necessary and we did not do it, we can only use didn’t need to.

• I didn’t buy any groceries because Sarah told me I needn’t have bought anything.
• I didn’t buy any groceries because Sarah told me I didn’t need to buy anything.

Be able to, be allowed to, be permitted to, be supposed / meant to, had better

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Be able to / be allowed to

We can use subject + be able to / be allowed to instead of can to express permission or possibility. (⇒ See Can, could, be able to – ability and possibility)

• We were allowed to eat all that we wanted. 
• You won’t be able to finish before the deadline. 

We do NOT normally use it + be able to/be allowed to + infinitive.

• It is allowed to use mobile phones.
• You are allowed to use mobile phones.
• Using mobile phones is allowed.

Be (not) permitted to

We can use be (not) permitted to +infinitive to express permission or prohibition in formal or official situations, to say what the rules or laws are. The form it + be (not) permitted to +infinitive can be used.

• It is not permitted to take photos of the archive documents. 
• Picnics are not permitted in the park. 
• Employees are permitted to use mobile devices. 

Be supposed to / be meant to

We can also use be supposed/meant to + infinitive to express obligation or permission, to say what we should or shouldn’t do.

• We are supposed/meant to check in one hour before take-off.  
• What are you doing? You aren’t supposed/meant to be here.

Had better, had better not

We use had better + infinitive (without to) to talk about actions we think someone should or shouldn’t do. There is often a negative result if the action is carried out. We normally use the shortened form ‘d better, and the negative form is  ‘d better not (hadn’t is NOT used).

• We’d better hurry up or we’ll miss our train.  
• You’d better not tell her you broke the vase –she’ll get very mad.
• You hadn’t better tell her you broke the vase.