Modals of Obligation and Necessities

Obligation and Necessities

We use must, have to or need to talk about obligations and necessities.

For example…

John must finish his homework before he goes out. (It’s an obligation for John.)

The negative of must is quite different. It means don’t do it. It is used to talk about prohibitions. “Have got to” has a very similar meaning with “have to”. It expresses necessity or obligation but it is soon or now. The difference between “need to” and “have to” is very subtle. In most cases they are interchangeable. “Need to” expresses a necessity where there are some choices it is based on your will, but in “have to” you have no alternatives and it is not your idea to do it.

For example…

I need to drink some water. (I can drink it later.)

I have to drink some water. (I have no choice. I really need it)

She needs to be at the conference today. (It is necessary for her to be there. She can learn many things necessary for her.)

She has to be at the conference today. (She has no other alternatives for her. She is obliged to go to the conference. She can be a speaker there.)

What’s the difference between have to and must?

We can use both modal verbs to talk about obligations. In many circumstances, both will be ok to use. However, “must” expresses an internal obligation “have to” expresses an external obligation. So we prefer to use “have to” when we talk obligations based on laws, rules or the obligation comes an authority. However, have to is also used with internal obligations. On the other hand, we can’t use must with external obligations.

For example…

I must tidy my room. It looks really messy.

I have to tidy my room. My mum has told me to do it.

I must/ have to go to the dentist.

Men have to do military service in many countries. It’s in their constitution.

What about negatives?

The negative form of “must” expresses a prohibition. You can replace “mustn’t” with “don’t do it” to see if it fits.

for example…

You mustn’t swim in the lake.  It’s very dangerous. (Don’t swim the lake. It is very dangerous.)

You mustn’t play music loudly when your sister is sleeping. (Don’t play music loudly. You can wake your sister up.)

The negative meaning of “have to” 

The negative meaning of “have to” is very different from its positive form. It means a lack of necessity along with not prohibition.

For example…

You don’t have to study tonight. The exam is next week. (There isn’t a necessity to study tonight. You can study if you want)

The students don’t have to go to school at the weekends. (It is not necessary to go to school at the weekend as there are no lessons at the weekends. However, “you mustn’t go to school at the weekends would express a prohibition. It would be wrong to use mustn’t as it is not prohibited to go to school at the weekends.)

Al doesn’t have to finish his project now. The deadline is next month. (He can finish today. There is no prohibition.)

Al mustn’t finish his project tomorrow. The deadline is today. (It is not OK to finish tomorrow. There is a prohibition. The last day to submit the project is today. )

“Not have to” is usually compared with “mustn’t”. You can check your answer by asking “Is there a prohibition to do it?”

What’s the difference between can’t and mustn’t?

You can also use can’t to talk about something not allowed or prohibited.  It expresses that it is against the rules. “Mustn’t” expresses a prohibition that comes from the speaker.

For example…

You can’t take pictures in the museum. (Don’t take pictures in the museum. It’s not allowed.)

You mustn’t pass when the traffic light is red. (Don’t pass when the traffic light is red. It’s against the law/rules.) 


Practice 1

Practice 2

Practice 3

Practice 4

Modals of Obligation And Necessities

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