Modal Verbs: Must

The Forms, Meanings and Use of the Modal Verb MUST.

Must is a modal auxiliary verb. 'Must' has no infinitive or participles. It  has only one tense form in the present. Like other modal verbs, 'must' has no –s on the third person singular (he must, not he musts); questions and negatives are made without do (Must I?, not Do I must?); 'must' is followed by the infinitive without to (e. g.: I must do it).

Patterns. Read and memorize! 

Affirmative	Interrogative	Negative
I must do it now.	Must I do it now?	I must not do it now.
He must do it now.	Must he do it now?	He must not do it now.
She must do it now.	Must she do it now?	She must not do it now.
It must do it now.	Must it do it now?	It must not do it now.
We must do it now.	Must we do it now?	We must not do it now.
You must do it now.	Must you do it now?	You must not do it now.
They must do it now.	Must they do it now?	They must not do it now.

Note 1: - The missing tenses are supplied by the past and the future tenses of to have.
E. g. : I had to meet him at the station.
          I shall have to meet him at the station.

Note 2: - There is a contracted negative form mustn’t [mʌsnt].
E. g.: He mustn’t find out what’s happening.

Meanings

The modal verb must expresses:
1). duty, obligation

Key examples:

1. Lizzie must see a doctor.       Must Lizzie see a doctor? 
2. Lizzie must keep to bed.       Must she keep to bed? 
3. Lizzie must take some medicine.      Must she keep to bed? 
4. Lizzie must follow the doctor’s advice.
5. Lizzie must take her temperature every evening.
6. Lizzie must gargle every two hours.

Note: - When denoting obligation the verb 'must' has two equivalents – to have and to be. The modal expression to be + infinitive is used when something is planned or thought of as unavoidable. The modal expression to have + infinitive denotes obligation or necessity arising out of circumstances.
E. g. : I am to make a report tomorrow so I have to go to the library.

to be + infinitive

Examples:

1. Jane is to make a report in December.
2. Jane is to take her exam in January. 
3. Jane is to be back at the University be the 7th of February.

to have + infinitive

Examples:

1. Larry has to go to the station to meet his friend. 
2. Larry has to hurry as there is little time left. 
3. Larry has to take a taxi.

► In informal British English, you can say have got to instead of have to. It can only refer to single events, and cannot be used to refer to the past:

• I’ve got to catch the bus at 6.30 tomorrow morning. 
• Have you really got to catch the bus at 6.30 tomorrow?

 but 

• Did you really have to catch the bus at 6.30 yesterday morning?

 2). prohibition (must not)
Examples:

1. Lizzie must not go out.  
2. Lizzie must not work these days. 
3. Lizzie must not disregard the doctor’s advice. 
4. Lizzie must not eat ice-cream.

Note: - If the absent of necessity is to be expressed, need not is used.

1. Lizzie need not (needn’t) go to the chemist’s, her brother will do it. 
2. Lizzie need not go to the doctor’s again. The doctor will come to her place. 
3. Lizzie needn’t worry about her work. She will catch up.

3). supposition, deduction

must + Indefinite Infinitive (or Continuous Infinitive)

In this case 'must' is used to say that we are sure about something (because it is logically necessary).

Examples:

1. Jack must be ill. He looks very pale.
2. Jack must be waiting for the doctor. He is in the reception room. 
3. Jack must be ill at ease. He looks worried. 
4. Jack must be tired. He looks exhausted. 
5. You must be Alisha’s mother. She looks just like you. 
6. Alisha must have some problem: she keeps crying.

Note 1: - The Indefinite (or Continuous) Infinitive refers the action to the present.
Note 2: - 'Must' is only used in this way in affirmative sentences. In questions and negatives, we use can and can’t instead.
E g: 'There’s the doorbell. It must be a postman.' – 'That can’t be the postman – it’s only seven o’clock.'

must + Perfect Infinitive

Examples:

1. Robert must have made a report at yesterday’s meeting. I saw the agenda. 
2. Robert must have spoken about the results of his research. 
3. Robert must have been a success. He looks pleased. 
4. Robert must have worked hard. He is very tired now. 
5. Robert must have seen Alice after the meeting. 
6. We’re lost. We must have taken the wrong road. 
7. Nick is behaving very strangely. I think he must have been drinking. 
8. 'We went to Majorca.' – 'That must have been nice.'

Note 1: - The Perfect Infinitive refers the action to the past (for deductions about the past).
Note 2: - 'Must' is only used in this way in affirmative sentences. In questions and negatives, we use can and can’t instead.

• Go to the 'Modal Verb May' page