How to Use Must in English

Some Essential Uses of Must, How to Use Must in English, Must is an important modal verb which we frequently use in our daily life.


Some Essential Uses of Must


Must is an important modal verb which we frequently use in our daily life. 

Definition : 

  • .Be obliged to; should (expressing necessity). “You must show your I’d card.”
  • Expressing an opinion about something that is logically very likely. "There must be something wrong"

Affirmative (+) form:

Must comes before the verb and after the subject.

Structure: Subject + must + base form of verb + extension. 

She must be your sister. 

You must have lots of books. 

Note: Not  She must can be your sister. or This can must be your sister.

Negative (−) form:

Subject + mustn't / must not + base form ofverb + extension. 

There must not be any rubbish left. 

You must not use your phone after 11pm.

Note: We don’t use don’t/doesn’t/didn’t with must There doesn’t must be any rubbish left.

Question (?) form:

Must + subject + base form of verb + extension+? 

Must you make a mistake?  

Question tags: 

We can use must and mustn’t in question tags though obligation. 

 tags with must aren’t very common.

The house must be worth millions, mustn’t it?

Deductions and conclusions:

When we think carefully about facts, we often use must to express deductions and conclusions from these.

He is so small. (Fact)

He must not be more than 5 years old. (deduction/conclusion)

We use must have to talk about deduction in the past. They always refer to deduction, not  obligation.

Mita want to talk to Rubi on an important matter so she called her but she didn’t answered the phone. So she call her again today.

I called you yesterday around 3pm but you must have been out. 

We must have been in the tea garden then.

Spoken English:

In speaking, we very often express our reaction to what we hear using phrases such as that must be or that must have been.

A:She lives in Thailand now.

B:That must be amazing!

A:Twelve years ago Kevin and I went on a six week camping trip. 

B: That must have been fun.

Obligation and Necessity:

Must expresses strong obligation and necessity .

 Seat belts must be worn even in the back of the car.

There must be a minimum of two members of the company at the meeting.


Rules and Laws:

*We use must not to talk about what is not permitted.


    You must not park outside the entrance.

    You must not make noise after 9 o’clock.

*Must and must not often occur in public signs and notices indicating laws, rules and prohibitions:

[airline website information]

All passengers must present valid photo identification at check-in for all flights.

[bus company website notice]

Tickets must be retained for inspection, and must be produced for inspection on request by any authorised official of Bus Éireann. (Bus Éireann is the name of the Irish national bus company)

Invitations and encouragement:

We also use must to express polite invitations or encouragement:

You must come and see us soon.

You must try some of this chocolate cake. It’s delicious.

You must go and see that film.


We use the question form of must in criticisms:

Must you keep playing that terrible music?

Why must you mispronounce my name every time?


We can also use have got to when we make deductions or draw conclusions. Must is more common than have (got) to in this meaning:

That must be a fake!

That picture has got to be a fake!

Typical Errors:

We don’t use must to express obligation and necessity in the past. We use had to instead:

When she got home, she had to cook dinner before everyone arrived.

Not: When she got home, she must cook dinner before …

We don’t use must to make predictions about the future. We use will instead:

Don’t worry about our accommodation because I found a nice hotel which will be suitable for us.

Not: Don’t worry about our accommodation because I found a nice hotel which must be suitable for us.

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