2.2.- Modal verbs for prohibition.

Do you know the difference between prohibition and no obligation?

Mustn't is used to express prohibition, when something is forbidden.

• In England people mustn't drive on the right.
• You mustn't speak loudly in the library.

Can't/Couldn't is used to say that something is prohibited.

• You can't smoke in the staff room.
• Last year you couldn't wear a skirt at school.

Needn't is used to say that something is not necessary. We don't have to do it because it is optional. There is no necessity or absence of obligation.

• You needn't do so much exercise. You are quite thin.
• I needn't study at Easter.

So, what is the difference between mustn't and needn't?
Study these two sentences:

• Doctor to an ill child: "You mustn't get up today" (It is prohibited for him to get up because he is ill).
• Mother to son: "You needn't get up early this Friday because it is a holiday". (It is optional for the child to get up early or not. He can do what he wants).

Don't have to / Don't need to are also used to say that something is not necessary, it is optional.

• I don't have to wear a jacket at work. (it is optional, not obligatory).
• We don't have to get up early on Christmas Day.
• You don't need to come to work tomorrow. Stay at home.
  She won't need to study next term.

Note the verb Need to is a regular verb: To need, needed, needed. It uses auxiliaries for negative and questions. It has all tenses.

Needn't and don't need to are almost the same in the present. However, when we are talking about general necessity, we normally use don't need to:

• You don't need to pay for medical care in National Health Service hospitals.