Unit 9 Must and Mustn't

 

Modal verbs: 'Must'

" Must " is a modal helper . It is used mainly to indicate an obligation or prohibition (" must do something " or " must not do something ").

As an auxiliary verb it shares the characteristics of modals:

- It is followed by an infinitive verb without "to", except be able to, need to, have to and ought to.

- The third person singular does not have -s in the present simple.

- You do not need to add the auxiliary do / does for the question and negative form.

- It does not have compound or progressive times.

- It does not have an infinitive.

Affirmative Interrogative	Negative form	Short negative
must	must not	mustn't

Affirmative	Negative
I must do	I must not do	I mustn't do
You must do	You must not do	You mustn't do
I must do	I must not do	I mustn't do
She must do	She must not do	She mustn't do
It must do	It must not do	It mustn't do
We must do	We must not do	We mustn't do
You must do	You must not do	You mustn't do
They must do	They must not do	They mustn't do

Interrogative
Must I do?	Mustn't I do?
Must you do?	Mustn't you do?
Must I do?	Mustn't I have?
Must she do?	Mustn't she do?
Must it do?	Mustn't it do?
Must we do?	Mustn't we do?
Must you do?	Mustn't you do?
Must they do?	Mustn't they do?

 

For other verb tenses (for example, the future) you can use " to have to ".

I'll have to go to the doctor.

I will have to go to the doctor ( future ).

I had to go to the doctor.

Had to go to the doctor ( past).

Uses of "must"

 one To express obligation, moral duty or advice .

You must be back before midnight.

You must be back before midnight.

She must respect her parents.

You must respect your parents.

You must visit New York.

You must visit New York.

The difference with "have to" is that "must" is used to give orders or express a moral obligation of the person who speaks or orders and "have to" indicates that the obligation comes from external norms, a third person or a circumstance .

I must go to work.

I have to go to work.

I have to do what my parents say.

I have to do what my parents say.

 2 To express prohibition .

The negative form, " must not " expresses a prohibition . Instead, " don't have to " indicates that something is not necessary, that is, there is no obligation (it can also be expressed with "need not / needn't" ).

You mustn't eat in class.

You should not eat in class.

You don't have to come to visit me.

You needn't come to visit me.

You don't have to come visit me.

 3 With meaning of affirmative deduction .

Only "must" is used.

I have arrived late. He must be tired.

He arrived late. You must be tired.

Look at that Porsche. It must be expensive.

Look at that Porsche. It has to be expensive.

For the negative deduction or impossibility we will use "can't".

He can't be her boyfriend.

It can't be her boyfriend.