Modal verbs of obligation

Obligation is when someone tells you to do something or gives you an order to do something. We can also tell ourselves to do something. We can use 'must / must not' for very strong obligations, like a law and also for weaker obligations like telling someone to do something.

• You mustn't use your phone when you're driving. (It's the law.)
• You must stop smoking. (=I'm telling you to stop smoking.)
• I must go home and study. (I'm giving myself an instruction or order here.)

We can also use 'have to' or 'have got to', with almost the same meaning.

• You've got to stop smoking.
• I have to go home and study.

Prohibition = it's not allowed

Can (permission) and must (obligation) have very different meanings in the affirmative ('you can do it' is very different from 'you must do it'). But they are quite similar in the negative, when they tell someone not to do something. In fact, US English often uses 'can't' in the same situation that UK English uses 'mustn't'.

• You mustn't smoke here.
• You can't smoke here.
• You mustn't go into the kitchen.
• You can't go into the kitchen.

Sometimes we also use 'may not' or 'be not allowed to' in the same situation. 

Let's review!

1. Obligation is when someone ___________ you to do something.
2. We can use ______________ and _____________ for strong and weak obligations.
3. Can (permission) and ___________ (obligation) have very different meanings in the affirmative.
4. We can use both __________ and 'must not' to tell someone not to do something.