38 Must: Modals of Obligation


A modal verb is a type of auxiliary (helping) verb that has no meaning on its own but it modifies the main verb, changes its meaning and gives more details about the action.

Must is a type of auxiliary modal verb used to express certainty, necessity or strong obligation which doesn’t come from outside, it’s a personal opinion. It also expresses prohibition in the negative form.


Must can be expressed in the three forms and it doesn’t change even for the third person singular.


Its structure, in the affirmative form, is:
Subject + must + verb + …

Subject must Verb
I/You must work
He/She/It must work
We/You/They must work


Its structure, in the negative form, is:
Subject + must not + verb + …

  • Short form of the negative form is: mustn’t.
Subject must + not Verb
I/You must not work
He/She/It must not work
We/You/They must not work


Its structure, in the interrogative form, is:
Must + subject + verb + …?

Must Subject Verb Question
Must I/you work ?
Must he/she/it work ?
Must we/you/they work ?


  1. Certainty/reasonably expected
    • It must be her flat. It’s number 13.
    • This house must be in a good state! They renovated it last year.
    • Must they be rich? They live in a luxury flat in the city center.
  2. Strong obligation
    • must search for a new house, my contract ends soon.
    • She must clean the kids’ clothes, they got very dirty at the park.
    • Must we be on time?
  3. Necessity
    • I must buy a new fridge.
    • He must search for a student flat.
    • Must they paint the walls?
  4. Prohibition (negative)
    • You must not give your address on online forums!
    • He must not surf the internet while working!
    • You mustn’t smoke in this restaurant.


We use must when:

  1. We are sure about something, something is reasonably expected (affirmative);
  2. There is strong obligation (affirmative, interrogative);
  3. Something is necessary (affirmative, interrogative);
  4. Prohibition (negative).


The modal verb must expresses certainty, strong obligation, necessity or prohibition which doesn’t come from outside. Must can be expressed in the three forms, and it doesn’t change even for the third person singular.

The structures are:

  • Affirmative: Subject + must + verb + …
  • Negative: Subject + must not + verb + …
  • Interrogative: Must + subject + verb + …?

For example:
— Affirmative: “I must visit my parents soon.” = The subject feels obligation.
— Negative: “You must not try drugs.” = It expresses prohibition.
— Interrogative: “Must I do it now?” = The subject asks if he has to do it in that moment.

Let’s revise this content within the {Form} section. Take a look at the {Example} section that shows its use within a context.

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