These ideas can be expressed using modal auxiliary verbs.
To show strong obligation use must.
- You must reach here before 5 o’clock.
- She must quit her job.
- You must stop smoking.
- All workers must arrive for work by 9 a.m.
Use need to ask if something is absolutely necessary.
- Need I wait any longer?
- Need I apply again?
- 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 can’t go.
- Books must not be taken out of the library.
- Students may not smoke in the classroom.
- 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.
- He should work harder.
- They should make another attempt.
- She really ought to apologize.
- You might ask his opinion.
- You had better start saying thank-you.
- What shall we do?