Modal Verbs – Must

We use “must” in the following cases:

  1. To express an obligation, duty or command.
  2. To express an urgent situation.
  3. To express a strong possibility which may be as high as 90%.
  4. To express a strong suggestion.


All employees must wear safety glasses in this area. (Obligation)

All motorists must wear their safety belts. (Obligation)

All students must take the entrance exam before registration. (Obligation)

You look horrible, you must go to the doctor! (Urgent situation)

She is mad at me, I must call her to apologize! (Urgent situation)

It is getting very late, we must leave! (Urgent situation)

We must take immediate action before it is too late! (Urgent situation)

She must be rich, she’s been working sixteen hours a day! (Strong possibility)

They must be madly in love, they are always kissing each other. (Strong possibility)

What time is it?

It must be midnight, I’ve just heard the church bell (Strong possibility)

We must go to the concert, it is going to be great! (Strong suggestion)

You must see that movie, it is very funny! (Strong suggestion)

You must go on a diet, you’re getting fat. (Strong suggestion)

Note: Never use “TO” with “MUST”.

Error: You must to go to church every Sunday.

Correct: You must go to church every Sunday.

Error: Children must to respect older people.

Correct: Children must respect older people.

Error: We must to finish painting the house.

Correct: We must finish painting the house.

The structure to use “must” is as follows:

Subject + Must + Verb in Infinitive Form + Complementizer
must study for the exam.
must go visit your friend in the hospital.
must buy some flowers for his wife.
must be at home, she never goes out.
must be sunny, it is hot in here.
must talk to the president.
must arrive on time tomorrow.

The negative form of “must” is must not or the contracted form “mustn’t” which is more commonly used as a negative command and not used commonly as a negative possibility.


Children mustn’t play with matches.

You mustn’t forget to mail the letter.

She mustn’t treat her boyfriend that way.

You mustn’t drink and drive.

They mustn’t play in the street after dark.

They mustn’t be there, they just left five minutes ago.

It mustn’t be raining, the sun is out.

It mustn’t be that late already!

He mustn’t be going out with her, she is too beautiful for him.

They mustn’t be sleeping still, it’s twelve o’clock.

Note: When expressing a strong negative possibility, it is much more common to use “couldn’t” rather than “mustn’t”.


They couldn’t be there yet, they just left 5 minutes ago.

It couldn’t be raining, the sun is out.

It couldn’t be that late already.

He couldn’t be going out with her, she is too beautiful for him.

They couldn’t be sleeping still, it’s twelve o’ clock.