Necessity: must, have to, mustn't and needn't.

A Present, past and future

We use must and have to/has to to say that something is necessary.

You'll be leaving college soon. You must think about your future.

We're very busy at the office. I have to work on Saturday morning.

Mark has to get the car repaired. There's something wrong with the brakes.

When we use the past, or the future with will, we need a form of have to.

Emma had to go to the dentist yesterday, NOT She must go to the dentist yesterday

That wasn't very good. We'll have to do better next time.

And in other structures we also use a form of have to, not must.

To-infinitive: I don't want to have to wait in a queue for ages.

After a modal verb: Emma has toothache. She might have to go to the dentist.

Present perfect: Mark has had to drive all the way to Glasgow.

For negatives and questions with have to/has to and had to, we use a form of do.

I don't have to work on Sundays.

Why does Andrew have to study every evening?

Did you have to pay for your second cup of coffee? ~ No, I didn't.

I don't have to work means that it is not necessary for me to work.

B Must or have to?

Both must and have to express necessity, but we use them differently.

MUST

We use must when the speaker feels that something is necessary.

You must exercise. (I'm telling you.)

We must be quiet. (I'm telling you.)

I/we must can also express a wish.

I must buy a newspaper. I want to see the racing results.

We must invite Claire. She's wonderful company.

HAVE TO

We use have to when the situation makes something necessary.

I have to exercise. (The doctor told me.)

We have to be quiet. (That's the rule.)

I have to buy a newspaper. The boss asked melt get one.

We have to invite Trevor and Laura. They invited us last time.

C Have got to

Have got to means the same as have to, but have got to is informal. We use it mainly in the present.

I have to/I've got to make my sandwiches.

My father has to/has got to take these pills.

Do we have to apply/Have we got to apply for a visa?

D A Mustn't or needn't?

We use must to say that something is necessary.

You must be careful with those glasses.

I must remember my key.

Now compare mustn't and needn't.

MUSTN'T

We use mustn't to say something is a bad idea.

You mustn't drop those glasses. They'll break.

I mustn't forget my key. or I won't get in.

You mustn't wear your best clothes. You'll get them dirty.

NEEDN'T

We use needn't when something is not necessary.

You needn't wash those glasses. They're clean.

We needn't make sandwiches. There's a cafe.

You needn't wear your best clothes. You can wear what you like.

E Don't have to and don't need to

We can use don't have to and don't need to when something is not necessary. The meaning is the same as needn't.

You don't have to / don't need to wash those glasses. They're clean

Mark doesn't have to /doesn't need to finish the report today. He can do it at the weekend.

For the past we use didn't.

The food was free. We didn't have to pay/We didn't need to pay for it.

C Didn't need to or needn't have?

Daniel hadn't booked a seat, but luckily the train wasn't full. He didn't need to stand.

Trevor and Laura booked a table for dinner. But the restaurant was empty. They needn't have booked a table.

DIDN'T NEED TO

We use didn't need to when something was not necessary. Standing was not necessary because there were seats.

Mark didn't need to hurry. He had lots of time. He drove slowly along the motorway.

We didn't need to go to the supermarket because we had plenty of food.

Sometimes we can use didn't need to when the action happened, even though it was not necessary.

Mark didn't need to hurry, but he drove at top speed.

NEEDN'T HAVE

We use needn't have + a past participle for something we did which we now know was not necessary, e.g. booking a table.

Mark needn't have hurried. After driving at top speed, he arrived half an hour early.

We needn't have gone to the supermarket. We already had a pizza for tonight.