Asking for permission
CAN and MAY are used when asking and giving permission, it is also used as a polite way to ask for something. MAY is the most formal one.
→ Can I sit here?
→ You can put your shoes right there
→ Can you schedule a class for me?
→ May I take your order?
→ These pages may be used by the students.
Obligation
We use MUST, NEED TO and HAVE TO for obligations [in affirmative sentences]:
→ You must finish your homework before next class.
→ I must finish my report before noon.
→ The doctor said I need to quit smoking.
→ I need to exercise more often.
→ She has to deliver the project before the deadline.
→ We all had to bring something to eat.
No obligation
and we use DON’T HAVE TO / NEED TO when it is not necessary or not an obligation
→ You don’t need to come with us if you don’t feel like doing it.
→ We don’t have to dance at the party
→ She doesn’t have to speak at the meeting if she’s busy.
Prohibition
In negative sentences we use MUSTN’T for prohibitions, to say it is necessary to not do something
→ You mustn’t use your phone while driving
→ You mustn’t make any noise in the library
→ She mustn’t eat that, she is allergic!
We can also use COULDN’T when talking about things that were forbidden (in the past) and CAN’T in the present, usually when talking about something against the rules.
→ We couldn’t say anything in the meeting
→ He said she couldn’t share the information with the team
→ I can’t park here
→ You can’t take photos in the museum.
Suggestions & Advises
Now, when advising someone, which means, making suggestions about a subject in order to help, we must use SHOULD and SHOULDN’T
→ You should see a doctor
→ You shouldn’t eat that amount of sugar if you wanna be healthy
→ She shouldn’t procrastinate her studies if she wants to become fluent
We can also use COULD for affirmative sentences when making suggestions
→ We could go somewhere else if you like
→ She could try studying in the morning
EXERCISES
- Fill the blanks using the most suitable word:
I've reserved the hotel for two nights. I _____ (must/had to) give them my credit card number.
I've reserved the hotel for two nights. I had to give them my credit card number.
When I was a child, we ____ (shouldn’t /couldn’t) watch television except at the weekend.
When I was a child, we couldn’t watch television except at the weekend.
You _____ (couldn’t/ mustn’t) smoke in here. You'll set off the fire alarm!
You mustn’t smoke in here. You'll set off the fire alarm!
When I was at school, we ____ (had to / must) stand up whenever a teacher came into the room.
When I was at school, we had to stand up whenever a teacher came into the room.
Children _____ (couldn’t / mustn’t) run inside the school building. They _____ (should / must) walk at all times.
Children mustn’t run inside the school building. They must walk at all times.
You can sleep here tonight, but you ____ (could / need to) leave tomorrow morning.
You can sleep here tonight, but you need to leave tomorrow morning.
In Roman times, ordinary people ____ (mustn’t / couldn’t) wear purple clothes.
In Roman times, ordinary people couldn’t wear purple clothes.
- Answer the questions:
→What must one do in order to gain weight?
→ How should one improve their speaking skills?
→ What could one do to be a healthier person?
