Obligation and Necessity

1. Obligation

We can use should, ought to, must, mustn’t, need (to), have to, and have got to when we want to express obligation.

2. Must and have to

must and have to/have got to

We prefer must:

to give orders and instructions

to show the speaker feels strongly about something

in public notices

We prefer have to:

to talk about rules and regulations

to talk about habits

have got to is more informal than have to.

I must go home now. (It’s my decision.)

I have to work tomorrow. (It is outside my control.)

3. Must and don’t have to

mustn’t means it is forbidden/prohibited. We can also use can’t to express prohibition.

don’t have to means it is not necessary.

Synonyms of don’t have to:

don’t need to

needn’t

haven’t got to

You mustn’t play ballgames in the park. (It is not allowed.)

You can’t eat all the cakes! Leave some to the others, too.

You don’t have to phone the client. (It is not necessary.)

4. Should and ought to

should and ought to are used to express advice, opinion or a polite instruction. In the negative we say shouldn’t and oughtn’t to.

You should eat more vegetables. (In my opinion is it good for you but you don’t have to do it.)

You ought to see the doctor. (My advice is to see the doctor but you have a choice.)

You shouldn’t work so much.

5. Had better and to be to

We can use had better or had better not to give strong advice.

You had better leave now.

He had better not be late!

We can use to be to when we want to give a formal instruction.

You are to arrive in the office at 8am sharp.

You are not to enter the premises.

6. Need (to)

In the present, we can use need as a main verb or as a model verb. It means ‘necessary.

I need to call mum. (main verb) – I need call mum. (model)

I don’t need to call her. – I needn’t call her.

Do you need to call? – Need you call?

In the past tense we use the main verb form with ‘to’:

I needed to call. I didn’t need to call.

But: needn’t have done has a different meaning:

You needn’t have brought the umbrella. (You brought the umbrella although it was totally unnecessary.)

7. Obligation in the past

When we talk about obligation in the past, we use had to or needed to. In the negative we use didn’t have to or didn’t need to.

I had to work last weekend. = I needed to work last weekend.

(It was necessary, and I did it.)

I didn’t have to work yesterday. (It was not necessary.)

If something was necessary but the person didn’t do it, we use: should have done or ought to have done.

I’m angry with Peter. He should have told me the truth! (Although it was important, he didn’t tell me the truth)