Foundation grammar · Unit 31 · A2

must, mustn't, don't need to

Modals, imperative etc.

Use must when something is necessary, had to for the past, mustn't when it is necessary not to do something, and don't need to when something is not necessary.

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A must + infinitive

Two friends walk past a new café. One says: Their cakes are amazing. You must try one.

The form is must + infinitive (must do, must see etc.):

Subject must Infinitive
I / we / you / they / he / she / it must do, stop, go, write etc.
  • You must try the cakes.
  • I must go now.
  • She must stop working so late.

B must: it is necessary

Use must when you think it is necessary to do something.

For the past (yesterday, last week etc.) we use had to, not must.

  • The car is filthy. I must wash it.
  • That book is brilliant. You must read it.
  • We must buy some bread today. There's none left.
  • We had to buy some bread yesterday. (not 'We must buy ... yesterday')
  • I had to get a taxi home last night. The last bus had gone. (not 'I must get')

C mustn't: it is necessary not to do it

mustn't = must not.

I mustn't do something means that it is necessary not to do it. It is the wrong thing to do.

A grandmother and a small boy are in a museum. The boy reaches out towards an old vase. She says: You mustn't touch it.

  • I must hurry. I mustn't miss the train.
  • I mustn't forget to ring the dentist. (= I must remember to ring the dentist)
  • You mustn't worry. Everything will be fine. (= don't worry)
  • You mustn't touch the vase. (= don't touch it)

D don't need to: it is not necessary

I don't need to do something means that it is not necessary to do it.

You can also say needn't (= need not). I needn't do it = I don't need to do it.

Compare don't need to and mustn't:

don't need to mustn't
You don't need to go. You can stay here if you want. You mustn't go. You must stay here.
  • I don't need to wash the car. It isn't very dirty.
  • You don't need to buy any bread. I got some this morning.
  • I needn't wash the car. (= I don't need to wash it)
  • You needn't buy any bread. (= you don't need to buy any)

Exercises

31.1 Complete the sentences. Use must + one of these verbs.

bebuychargemeetphoneseevisit

  1. My phone is nearly dead. I must charge it.

    My phone is nearly dead. I must charge it.

  2. My cousin Dan is very funny. You him.

  3. There's no milk left. I some on the way home.

  4. You the castle while you're in Conwy. It's beautiful.

  5. I my mum tonight. It's her birthday.

  6. The interview tomorrow is very important. I on time.

  7. That new comedy is brilliant. You it.

31.2 Put in I must or I had to.

  1. I had to get the bus to work yesterday. My car wouldn't start.

    I had to get the bus to work yesterday. My car wouldn't start.

  2. It's nearly midnight. go to bed.

  3. I don't usually cook, but last night make dinner for six people.

  4. remember to buy a card for Ellie. Her party is tomorrow.

  5. I flew to Belfast last week. The flight was late and wait four hours at the airport.

  6. My alarm didn't go off this morning, so get dressed in two minutes.

  7. I didn't answer Kirsty's message yesterday. reply to her today.

31.3 Complete the sentences. Use mustn't or don't need to (needn't) + one of these verbs.

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  1. I've already made a pie for tonight. You don't need to cook anything.

    I've already made a pie for tonight. You don't need to cook anything.

  2. The film doesn't start until eight. We .

  3. This plate belonged to my great-grandmother. You it.

  4. Go home if you're tired. You for me. I'll get a taxi later.

  5. We to lock the back door tonight.

  6. It's only a barbecue in the garden. You a suit.

31.4 Find the sentences with the same meaning.

  1. We must leave the dog at home.
  2. We couldn't leave the dog at home.
  3. We can't leave the dog at home.
  4. We don't need to leave the dog at home.
  5. We can leave the dog at home.

  1. We can take the dog with us.d

    We can take the dog with us. We don't need to leave the dog at home.

  2. We must take the dog with us.

  3. We mustn't take the dog with us.

  4. We don't need to take the dog with us.

  5. We had to take the dog with us.

31.5 Put in must, had to, mustn't or don't need to.

  1. You don't need to come with me. You can wait in the car if you like.

    You don't need to come with me. You can wait in the car if you like.

  2. This curry is amazing. You must try it.

    This curry is amazing. You must try it.

  3. There's plenty of food in the freezer, so we go to the supermarket.

  4. The freezer was empty yesterday, so we go to the supermarket.

  5. I want to hear all about your trip. You tell me everything.

  6. You tell Gran about the broken window. I don't want her to know.

  7. I leave now, or I'll miss my train.

  8. "Why are you so tired?" "I get up at five this morning."

  9. We book a table. The café is never busy on Mondays.

See also