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Present prohibition with should / ought to
It is even less common to use ought to in the negative form. It is grammatically possible, but the
construction is a little wordy and awkward.
You shouldn’t / ought not to go to work today if you feel so ill.
You shouldn't / ought not to do things like that. People don’t like it!
Present prohibition with can’t
It is also possible to use can’t to express prohibition, though it is not as strong as when you use mustn’t.
We also tend to use can’t to talk more about something that is not possible. Let’s contrast these two
sentences:
You can’t go out tonight. You’ve got homework to do for school tomorrow!
You mustn’t go out tonight. You’ve got homework to do for school tomorrow!
Obligations and prohibitions in the past
Again here, some semi-modals come into use – especially when using the past form of must.
Must (present) ? had to (past)
Should (present) ? should have (past)
Don’t have to (present) ? didn’t have to (past)
Shouldn’t (present) ? shouldn’t have (past)
Can’t (present) ? couldn’t (past)
5. Modals of Permission – can, may, might, could, must
Permission, says the Macmillan dictionary, is the right to do something that is given to you by someone
in authority. Can and may are the most commonly used modal verbs when asking for permission. May is
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used much less these days and sounds politer and more formal.
You can go out when you’ve finished your dinner.
Can I borrow your car at the weekend, Dad?
Use a black pen to complete the form, but you may also use a blue pen if you do not have one.
May I ask what your full name is, please?
Using might for permission is possible, but it is very rarely used outside formal situations, and even then
is used only in questions. Might I can also be used to make a suggestion in a very formal and polite way.
Some people think of might as a more formal version of may.
Might I recommend the grilled salmon fillet to go with your white wine? (posh, formal)
We can also use must not or mustn’t to talk about permission. Of course, this is when we are not giving
permission. These modals are more formal and are often used on signs and in official announcements.
Passengers must not speak to the driver while the bus is moving.
You mustn’t smoke in this restaurant – it is not allowed here!
Children under 10 must not use the swimming pool without adult supervision.
In addition, we use could to ask for permission, and it is a more formal version of can.
Could I have some more orange juice, please?
To talk about permission in the past, we can only use the modal verb could. But it is also possible to use
semi-modal forms in the past:
Can (present) ? could (past)
May / might (present) ? was / were allowed to (past)
Some examples of this in use:
Both staff and students could use the ice rink.
We couldn't study in the library after 6pm.
I couldn’t ask any questions until the end of the lecture.
Although I was a little too young, I was allowed to enter the race.
My tutor’s name was Robert, but we were allowed to call him Rob.
Similarly, we can also use these semi-modals to talk about the present:
I'm sorry sir, you are not allowed to go backstage without a pass.
My dog is not allowed to jump up or sit on the sofa.
Be careful when using modal verbs in the negative form. Avoid using don’t and didn’t.
I don’t should drink so much coffee at work. 
(Wrong)
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