Talking about duties and freedom to act

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?


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.