Modals of Obligation – Must

Verbs

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 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. In its Negative form it expresses prohibition.

Affirmative

The Affirmative form of the verb Must has this structure:
Subject + must + verb.

PERSON MUST VERB
I must take
You must take
He must take
She must take
It must take
We must take
You must take
They must take

Negative*

The Negative form of the verb Must has this structure:
Subject + must + not + verb.

PERSON SHOULD NEGATION VERB
I must not take
You must not take
He must not take
She must not take
It must not take
We must not take
You must not take
They must not take

*The Negative form usually expresses prohibition.

***Short form of the Negative form is: “mustn’t‘”

Interrogative

The Interrogative form of the verb Must has this structure:
Must subject + verb + (?).

MUST PERSON VERB QUESTION MARK
Must I take ?
Must you take ?
Must he take ?
Must she take ?
Must it take ?
Must we take ?
Must you take ?
Must they take ?
  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 5 bedroom 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 on the net 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).

We use the modal verb Must to express certainty, necessity or strong obligation which doesn’t come from outside. Must can be expressed in the three forms (Affirmative, Negative and Interrogative).

The structure for Must in its three forms is:

  • AffirmativeSubject + must  + verb (e.g. I must visit my parents soon.);
  • Negative: Subject + must + not + verb (e.g. You must not try drugs.);
  • Interrogative: Must subject + verb + (?) (e.g. Must I do it now?).

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

NOTE: The form is always the same, even for the third person.

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