Modals expressing obligation

There are 6 modals which express obligation: must, have to/have got to, be to, shall, should, ought to.

6.15.1.1. Must

Must expresses strong moral obligation, necessity, determination. For example: / must work. (=1 need to work. I am determined to work. Nobody makes me work.)

Grammatically must as a modal expressing obligation is the present tense form that has no counterparts in the past and future. Therefore the idea of must in those tenses is rendered by means of have to. She felt she had to have the air. We are sure we'll have to forgive him.

Note that in indirect speech must is used to refer an action to the past: The doctor told me I must stop smoking.

Must is always associated with the infinitive without to and has no -s suffix in the third person singular: This information must be given to the general public.

Must does not take any auxiliary to form a question or a negation. The negative forms of must are 1) must not/mustn't which expresses prohibition and 2) need not/ needn't that denotes unnecessary action. Compare: You mustn't smoke in the classroom. (=It is forbidden to smoke here.) — You needn't arrive at the airport till 10.30. (=It is unnecessary to arrive at the airport till 10.30.)

6.15.1.2. Have to/have got to  "

Have to/have got to expresses obligation arising out of circumstances: / have to work now. I have got to go now. (= I am obliged to.) / had to get up early yesterday. I'll have to meet him at the station tomorrow.

Have always requires the infinitive with the particle to.

In the third person singular of the present tense have to is used in the form of has to: The work has to be done by tomorrow.

As a modal have to always takes the auxiliary do to form questions and negation.

Compare:

Do we have to stay after the classes? — No, you don't have to stay after the classes.

Have we got to stay after the classes? — No, you haven't got to stay after the classes.

Does she have to get to the airport in an hour? — No, she doesn 't have to get to the airport in an hour.

Has she got to get to the airport in an hour? — No, she hasn 't got to get to the airport in an hour.

Did they have to learn the poem by heart? — No, they didn 't have to learn the poem by heart.

6.15.1.3. Be to

Be to expresses obligation of a pre-planned character or mutual arrangement: She was to meet him at five o'clock sharp. The train is to arrive at nine o'clock p.m. We are to be married in June.

As a modal be always takes the infinitive with to.

In the third person singular of the present tense be is used in the form of is: The child is to be in bed at 8 о 'clock.

Be does not require do as an auxiliary to form ques­tions and negative sentences. For example: She is not to be my friend. Was he to stay here for long?

6.15.1.4. Shall

Shall is used to express moral obligation, duty, com­mand. For example: Shall I open the window? (= Do you want me to open the window? ) Shall the boy wait?

In this meaning shall is often used in formal writing: You shall not kill. (The Bible) Whoever commits robbery shall be punished with rigorous imprisonment and shall also be liable to a fine. Payment shall be made by the end of the month.

In the negative shall expresses prohibition: You shall not have it! It's mine. He says he will do it but I say he shall not (do it).

6.15.1.5. Should

Should is used to express mild obligation in the form of advice or recommendation: If you see anything unusual you should call the police. You should practise more to become a professional musician.

To refer an action to the past should takes a perfect infinitive: You should have told me this long ago. He shouldn 't have said this.

The negative form of should is used to warn that an action is wrong or unwise. For example: He shouldn't be so impatient with people. You shouldn't talk so loud; you'll wake the baby. Compare with needn't which means that

something is unnecessary: You needn't talk so loud; I can hear you.

In modern English should is synonymous to ought to though should is milder in British English.

 

6.15.1.6. Ought to

The third person singular is ought. Its negative form is ought not/oughtn't. Ought always takes the infinitive with to.

Ought to is used to denote moral duty: She ought to look after her children better. You ought to be ashamed oj yourself.

To refer an action to the past ought takes a perfect infinitive: You ought to have helped him. This old coal ought to have been thrown away years ago.

The negative form of ought is used to warn against г wrong or unwise action: You oughtn 't to talk so loud, you'll wake the baby. Compare needn't which denotes an unnecessary action: You needn't talk so loud; you'll wake the baby.

Ought and should are similar in meaning but ought is slightly stronger in British English.

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