Modal Verbs: Permission, Prohibition, Obligation, No obligation 

We use modal verbs when we want to express permission, prohibition (not allowing something), obligation or no obligation. 

Permission – can, may, could

‘Can‘ is most often used to ask for or give permission, but ‘may‘ and ‘could‘ are also possible even though they are not used as often as ‘can’. 

• You can sit here; the seat is free. 
• Could I open the window? 
• May I ask a question? 

Prohibition

‘Can’t‘ and ‘mustn’t‘ (must not) are used to show that something is prohibited (not allowed). 

• You can’t go into that restaurant without a tie. 
• You can’t drive in this country unless you are over eighteen. 
• You mustn’t use your phone in class. 

‘Can’t’ usually gives the idea of something that is against the rules. ‘Mustn’t’ usually means that it is the speaker who is setting the rule. 

Obligation

‘Have to‘ and ‘must‘ are both used to express obligation. There is a slight difference in the way that they are both used. 

The difference between ‘have to’ and ‘must’

‘Have to’ shows that the obligation comes from someone else, not the speaker. This is usually referring to a rule or law. 

• We have to be at the airport at least two hours before the flight. 
• I have to work on Saturday. 
• They have to wear their uniforms at school. 

‘Must’ shows us that the obligation comes from the speaker. 

• I must hand in my thesis by tomorrow. 
• I really must call my parents. 

Let’s look at an example. Do you notice a difference in these two sentences? 

My doctor said that I have to stop smoking or I’ll risk serious problems. (I have no choice) 

I must stop smoking. It’s costing me too much money. (it’s my decision) 

No obligation

We use ‘don’t have to‘ to show that there is no obligation. You can do something if you want but it is not an obligation. 

• You don’t have to wear a tie to go to the restaurant, but it would be nice. 
• You didn’t have to drive me. I could’ve ordered a taxi. 
• Students don’t have to wear uniforms to school.