EXPLANATION OF MODALS: OBLIGATION & PERMISSION

Obligation and permission

When we want to talk about what we are obliged to do (or what we must do), we can use the verbs 'must' or 'have to'. To talk about what we are permitted to do (or allowed to do), we can use the verbs  'can' or 'be allowed to'.

Must and Have to: 

Must and Have to are both used to talk about rules or obligations. MUST suggests there's some kind of internal obligation whereas HAVE TO suggests some kind of imposed obligation. 1. Must and have to are both followed by the base form of the main verb: She must give money to her parents every month. They have to get a visa before they go on holiday to the USA. 2. Must is a modal verb so it doesn't change its form: I must wear a uniform. She must do her homework. 3. Have to isn't a modal verb so its form does change: I have to get up early for my job. He has to have short hair because he's in the army. 4. There is no past form of 'must', instead we use 'had to': We had to obey our teachers when we were students. She had to drive on the right when she was on holiday in France.

Mustn't and Don't have to:

1. Mustn't and Don't have to are not used in the same way. Mustn'tis used to talk about things we are obliged not to do: In most countries, you mustn't drink alcohol and then drive. It's against the law. You mustn't run in the corridors. You might slip. 2. Don't have to is used to talk about things we are not obliged to do, we can do if we want or not: It's up to you, we can go to the cinema or not. I don't mind. We don't have to go out if you don't want to. If you don't like the food, don't worry you don't have to eat it.

Can and be allowed to:

Can and be allowed to are both used to talk about permission. 1. Can and be allowed to are both followed by the base form of themain verb: In Britain you can drive when you are 17. He's allowed to wear jeans at his school. 2. Can is a modal verb so it doesn't change its form: I can stay out till 10.30, on Saturday nights but I have to be home by 8.30 during the week. You can't smoke here, Khalid. 3. Be allowed to isn't a modal verb so its form does change: I'm allowed to smoke. My parent don't mind. She is allowed to wear make-up, even though she's only 14. 4. The past form of 'can' is 'could' and of 'am/is/are allowed to' is 'was/were allowed to': We could eat as many sweets as we liked, when we were young. She was allowed to drive her father's car when she was only 15. However, we only use 'could' to talk about general permission. When we want to talk about a particular episode of permission in the past, we use allowed to: Last night she was allowed to have a bowl of ice cream because she had eaten all her vegetables.