These ideas can be expressed using modal auxiliary verbs.

Strong obligation

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.
• You must leave now.
• He must pay the fee.
• She must come.
• We must find a solution to this problem.

Use need to ask if something is absolutely necessary.

• Need I wait any longer?
• Need I apply again?
• Need I wait any longer?
• Need I pay for that call?
• Need I pay the dues?
• Need I call him?
• Need I invite them?

Prohibition

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.
• You must not smoke in the kitchen.
• 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.
• You really ought to do something about that.
• She really ought to apologize.
• You had better start saying thank-you.
• What shall we do?
• You might ask his opinion.