Mustn't Needn't



Mustn't And Needn't Explanation

You must do means  that it is necessary that you do something. Let's see the following examples.
  1. You have't got much time. You must hurry.
  2. You can tell John what I said but he must keep it a secret .

You mustn't do means that it is necessary that you do not do something. Let's see the following examples.
  1. The baby is is asleep. You mustn't talk loudly. 
  2. This is a non-smoking area. You mustn't smoke here.
You needn't do means that it is not necessary that you do something. Let's see the following examples.
  1. I can hear you quite clearly . You needn't talk loudly. (= It is not necessary to talk loudly.)
  2. You've got plenty of time. You needn't hurry. (=It is not necessary to hurry.)
Needn't do can be replaced with doesn't/don't need to do. So, the two sentences above can be rewritten this way:
  1. I can hear you quite clearly. You don't need to talk loudly.
  2. You've got plenty of time. You don't need to hurry.

Needn't Have Verb III 

We use needn't have done to say that it was not necessary to do (in the past). Let's see the following examples.
  1. Achmad wanted to catch a plane. He left home late, so he hurried to the station. But the train was also late - it didn't leave until 20 minutes after Achmad arrived at the station. So, Achmad needn't have hurried.
  2. Jane bought some sugar but when she got home she found that she already had a lot of sugar. So she needn't have bought any sugar.

Didn't Need To - Needn't Have

Didn't need to is different from needn't have.
We use didn't need to  to say that an action was unnecessary. This was known at the time of the action.  Let's see the following examples.
  1. I didn't need to go, so I didn't go.
  2. I did't need to go but I decided to go anyway.
When we use needn't have, it was not known at the time that the action was not necessary. Let's see the following example.
  1. I need't have gone. (= I went - this was unnecessary, but I didn't know at the  time that it was unnecessary.)     

Exercise

In this exercise you have to complete the sentences with needn't or mustn't !
  1. Jack gave me a letter to post. I ...........forget to post it.
  2. "Do you want me to wait?" "No, It's okay. You ...........wait."
  3. This ring is very valuable. You ........lose it.
  4. You .......come if you don't want to.
  5. We still have enough eggs. We ........buy any.

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