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)