Using modal auxiliaries to express ideas like obligation, prohibition etc.

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.

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?


Manjusha Nambiar

Hi, I am Manjusha. This is my blog where I give English grammar lessons and worksheets.

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