1. MODAL VERBS

2. CAN, MUST, SHOULD, OUGHT TO, HAD BETTER
Can / could, must, should / ought to are the most common modal verbs to talk about permission and obligation.
You can’t park here.
I couldn’t take any pictures.
If you want to pass the exam, you must study.
We should / ought to take the highway, it’s quicker.
May to ask for permission. May I use the phone?

3. CAN, MUST, SHOULD, OUGHT TO, HAD BETTER
Should have
Ought to have
To talk about past events which did not happen and we REGRET.
We should have taken the car. We’re going to be late.
You ought not to have told a lie. Now it’s too late.
Had better (had better not) is stronger and more urgent.
Strong advice or warning. Normally, immediate future.
You had better post the parcel today or it won’t get there in time.
+ PAST PARTICIPLE

4. MUSTN’T and DON’T HAVE TO
MUSTN’T: to express and obligation NOT to do something.
You mustn’t bring pets into the shop. It’s forbidden.
DON’T HAVE TO: to express the ABSENCE of obligation.
You don’t have to tip here unless you think the service was excellent.

5. NEED NEED / DON’T NEED + to + infinitive NEEDN’T + infinitive
To say that something is necessary / unecessary
Habitual, general and specific necessity.
You usually need to bring your book to class.
You don’t need to close the door. I always have it open.
NEEDN’T + infinitive
To say that something is uncessary on a specific occasion.
You needn’t lock the car. Nobody will steal it here.
(but if you park somewhere else, you need to lock it)

6. NEED NEEDN’T HAVE + PARTICIPLE DIDN’T NEED TO + INFINITIVE
To say that something WAS NOT necessary but you did it.
You needn’t have booked. The restaurant is empty!
You didn’t have to book. The restaurant is empty!
To say that something WAS NOT necessary so you DID NOT do it.
COMPARE:
We didn’t need to book. (Unecessary, so we didn’t book).
We needn’t have to book. (Unecessary, but we did book).

7. BE ABLE TO, BE ALLOWED TO, BE PERMITTED TO, BE SUPPOSED/ MEANT TO
PERSON + BE ABLE TO or BE ALLOWED TO instead of can to talk about what is possible or permitted.
You aren’t allowed to use your phone in class.
NOT  It isn’t allowed to use…
IT + BE PERMITTED TO + INFINITIVE in formal situations. To say what can/can’t be done according to laws and regulatios.
It is not permitted to smoke inside the school.
BE SUPPOSED TO / BE MEANT TO + INFINITIVE to say what people should / shouldn’t do because of the rules. Implies the rules are not necessarily obeyed.
Students are not supposed to /meant to cheat in the exam, but some do.