Talking about obligation and freedom to act
We can use modal auxiliary verbs to talk about duties and freedom to act. Common modals used to express these meanings are: should, must and ought to.
To show strong obligation use must.
You must reach here before 5 o’clock.
You must mend your ways.
She must stop seeing him.
He must pay the dues.
You must obey the rules.
We must do something.
She must quit her job.
All workers must arrive for work by 9 a.m.
You must stop smoking.
Use need to ask if something is absolutely necessary.
Need I wait any longer?
Need I apply again?
Need I pay for that call?
Need I ask his opinion?
Need I pay the dues?
Need I call him?
We can use structures like may not, cannot and must not to prohibit people from doing something.
You must not write on the wall. (It is prohibited.)
You must not vandalize public property.
You can’t go.
Books must not be taken out of the library.
Students may not smoke in the classroom.
You must not cross the railway tracks.
You can’t come in.
Weak obligation; recommendation
These ideas can be expressed using the auxiliary verbs should, ought to, had better, might and shall.
You should consult a doctor.
You should buy a faster car.
He should work harder.
She should get a degree.
They should make another attempt.
She really ought to apologize.
What shall we do?
You might ask his opinion.
You had better start saying thank-you.