Modal Verb Must Grammar

Modal Verb Must Grammar - must be, must have

Must is used as a modal verb in the same way as the other modal verbs are used. "Must"comes after the subject and before the verb for example:

  • He must have many friends.

We use "mustn't" as negative form must.

  • She mustn't  phone him.

Note: We don't use can't/don't/doesn't/shouldn't/didn't  etc with must.

She mustn't doesn't phone him. (incorrect)

In the formal context the full form the must not can be used or we  use we want to emphasise something.

  • You must not make noise in the class.

Must Grammar

When we talk about facts we usually use must to show conclusion and deduction.


  • She doesn't talk to him now. She must be no longer his friend.

Note: In the first sentence we talk about (fact) and in other we talk about deduction first then conclusion.

More examples:

  • He looks little ill. (fact)
  • He can't pay attention to his health or he must work too hard. (deduction, conclusion)

Note:  In negative sentence we use can't or cannot in place of must when we talk about deduction or conclusion.

When we talk about deduction in the past, we always use must in the following way.

 must have + 3rd form of the verb  -  can't have + 3rd form of the verb

  • The teacher punished the students yesterday. The students must have bothered the teacher.
  • She joined us quite late. She can't have got free from the work early.

We also use must to show strong obligation, force and necessity.


  • You must prepare well for your exams.
  • There must be legal debate in the meeting.
  • He must play well to qualify for the final.

Note: When we talk about force, necessity and obligation in the past we use had not must.


  • We had to work a lot last month for the company.

Future In The Past

We use must to talk about future in the past.


  • Last year i resolved that i must play for the country.
  • Will have to is used more than must to talk about future obligation.


  • She will have to deal with all home chores.
  • I will have to submit my assignment next week.

See also Have to

Note: We also use have got to + 1st form of the verb to talk about obligation.

  • We have got to meet mother before she leaves for Washington.
  • You have got to pay attention to the teacher's lecture.

See also Have got to

"Must" is used more general references to the future.


  • Father must decide next week about visiting Chicago.
  • All friends must gather early in the morning for tomorrow's trip.

"Must" is also used to talk about polite invitations and encouragement.


  • You must visit my house and meet my mother.
  • You must go out with me and have some food.

We also use "must" to show criticism. Remember that it is only used in question form of must.


  • Must you keep behaving in this way?
  • Why must we discuss this useless topic for nothing?

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