We use modal verbs to show if we believe something is certain, probable or possible (or not). We also use modals to do things like talking about ability, asking permission making requests and offers, and so on.
1. Ability: Can, Could, Be able to
- Can you swim?
- I can’t open this bottle.
- She could swim when she was five.
- I’d like to be able to play the piano.
- I won’t be able to remember your phone number.
2. Permission: Can, Could, May, Be allowed to
Asking for permission:
- Can I borrow your pen, please?
- Could I use your phone, please?
- May I make a suggestion?
Talking about permission:
- You can/are allowed to drive a car in Britain when you are 17.
- When we were children, we could/were allowed to stay up late on Saturday nights.
3. Obligation and Necessity: Must, Have to, Have got to
There is sometimes a difference between "must" and "have to"
Must: when you say what you think is necessary, when you are giving your opinion.
Have to: when you are not giving your personal opinion, but just facts
- The government really must do something about unemployment.
- I must write to Ann. I haven’t written to her for ages.
- Ann’s eyes are not very good. She has to wear glasses for reading.
- At our school, we have to wear uniform.
In everyday speech, we use "have got to" instead of "have to"
- I’ve got to work late this evening
We only use "must" to talk about the present and the future. When we talk about past obligation or necessity, we use "had to"
- I had to work late yesterday.
Mustn’t = Prohibition
- You mustn’t drive without a licence.
Don’t have to: when it is not necessary to do something
- You don’t have to wash that shirt, it isn’t dirty.
Don’t have to = don’t need to, needn’t
- I needn’t/don’t have to get up early. It’s Sunday.
4. Possibility: May, Might, Could
To talk about present or future possibility
- “There is someone at the door.” “It may be Sarah.” (Perhaps it is Sarah)
- President Jones might win the election. (Perhaps he will win the election)
- “Where’s Simon?” “He could be in the living-room.” (Perhaps he is there)
Negatives: may not, might not/mightn’t, but not use “could not” with this meaning
- Simon may not be in the living-room.
Note the form: may/might/could + be + V-ing
- They may be having dinner at the moment. (Perhaps they are having dinner.)
5. Deduction: Must, Can’t
- He must know London very well. He has lived there for a long time. (I am sure that he knows London very well)
- There is a light in the house, so someone must be at home. (I am sure that someone is at home)
- She can’t be in Italy! I saw her today! (It is impossible that she is in Italy)
- You’ve just had lunch, you can’t be hungry. (It is impossible that you are hungry)
7. Advice: Should, Ought to ("Should" is more common than "Ought to")
- I think you should talk to your teacher about it.
- You ought to stop smoking.
- You shouldn’t tell lies.
We also use these verbs to say what we think is right or good
- I think the police should arrest hooligans.
- What do you think I ought to do?