Unit 7.1

Must: Modals of Obligation

Introduction

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.

Form

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

Affirmative

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

Negative

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

Subject must + not Verb
I/You must not work
He/She/It must not work
We/You/They must not work

Short form of the negative form is: mustn’t.

Interrogative

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

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

Example

Certainty/reasonably expected

  • It must be her dog. The collar says her name.
  • 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 centre.

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?

Necessity

  • must buy a new bunch of flowers.
  • He must search for a student flat.
  • Must they chop the olive trees down?

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.

Use

We use must when:

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

Summary

The modal verb must expresses certainty, strong obligation, necessity or prohibition which doesn’t come from outside. We can use it in its different forms (it doesn’t change in the third person singular):

We can use it in its different forms:

  • Affirmative: We start with the subject followed by must and the verb.
  • Negative: We start with the subject followed by must not and the verb.
  • Interrogative: We start with must followed by the subject and the verb (the sentence ends with a question mark).

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.

Exercises


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