This document summarizes the uses and meanings of several English modal verbs: Can is used to express ability, possibility, and permission. Could expresses ability and possibility in the past or with more formality. Must expresses obligation. Mustn't expresses prohibition. Have to expresses obligation, while doesn't have to expresses lack of obligation. Should and shouldn't are used to give advice or express opinions.

1. MODALS

2. CAN
•ABILITY TO DO SOMETHING.
•I can swim.
•I can’t speak Italian.
•I can jump over that bench.
•POSSIBILITY.
•Smoking can cause cancer.
•You can get serious accidents if you drive too fast.
•PERMISSION.
•Mum, can I go out after doing my homework?
•Can I use your phone, please?
•REQUESTS.
•Can you wait a moment, please?
•Can I have another biscuit,please?

3. COULD
•ABILITY TO DO SOMETHING IN THE PAST.
•I could swim when I was only three years old.
•I couldn’t speak Italian until I went to Rome on holidays.
•I could run marathons when I was younger.
•POSSIBILITY IN THE PAST.
•Being a knight could be very dangerous in medieval times.
•You could become very rich playing poker when I was young.
•PERMISSION, (MORE FORMAL)
•Mum, could I go out after doing my homework?
•Could I use your phone, please?
•REQUESTS (MORE FORMAL).
•Could you wait a moment, please?
•Could I have another biscuit,please?

4. MUST
•OBLIGATION
•You must study for your tests.
•I must go to the supermarket today.
•I must stop the car when the traffic lights turn red.

5. MUSTN’T
PROHIBITION.
You mustn’t smoke in a hospital.
You mustn’t eat in class.
You mustn’t work on dad’s laptop.

6. HAVE TO
OBLIGATION.
I have to write an e-mail to my boss right now.
I have to work every day.
Teachers have to be very patient.
Sportsmen have to train very hard.

7. DO(ES)N’T HAVE TO
LACK OF OBLIGATION (NO OBLIGATION).
They don’t have to study a lot.
I don’t have to work on Saturdays.
We don’t have to read “The Old man and the Sea”. It’s
optional.
You don’t have to eat that if you don’t want to.

8. SHOULD/SHOULDN’T
ADVICE.
I should see a doctor. I have a terrible headache.
You should warm up before training.
You should revise your tests.
You should drive carefully in bad weather.
OPINION.
Your car is very old. I think you should buy a new
one.
You are very tired. I think you should go home and
get some sleep.