The following quiz questions will give you a taste of the difficulties that non-native speakers have in learning this troublesome modal
Correct the mistakes in the use of must in the following sentences:
Explain the difference in meaning in the use of must in the following sentences:
How would you disagree with the speaker of the first of the following sentences?
What's the difference between these sentences?
Are all the following statements correct?
What's the difference between the following pairs of sentences?
Must must be replaced by a form of its near equivalent have to in all tenses other than the present.
The sentence You must be serious means I want you to be serious or It is important that you are serious. If it is important that you are not serious, then I say You must not be serious.
The statement You must be crazy means I think you are crazy. You must not be crazy does not mean I don't think you're crazy, it means It is important that you are not crazy.
The previous answer illustrated the two most common uses of must: to express obligation or necessity, and to express probability or certainty. In the sentences below, must is being used in its second sense. To disagree with the speaker or to express your own probability/certainty that something is not true, you must use can and not must: The correct answers are:
So when John McEnroe shouted You cannot be serious! at the umpire, it was a perfect expression of his certainty that the umpire was not doing his job.
She had to tell him means she was obliged to tell him, whereas She must have told him is an expression of my certainty that she told him.
The sentence in red below is incorrect.
I mustn't go means I am not allowed to go or It is very important that I don't go. To express the idea that I am not obliged to go, you must say: I don't have to go. No wonder German speakers often get this wrong since Ich muss nicht gehen means I don't have to go. (I am not allowed to go = Ich darf nicht gehen.)
The difference in the pairs of sentences below is a subtle one. If the speaker uses must, she implies that she herself believes that the action is important and should be done. If, on the other hand, she uses have to, she implies that the obligation has been imposed on her by someone else.
So, I must eat less junk food implies that the decision is mine, whereas I have to eat less junk food implies that someone else (for example, my doctor) has told me what to do. Similarly, You must call me when you arrive suggests that I think it is very important for you to call me, while You have to call me when you arrive implies that it is a regulation (for example, of the company that you work for) that you call on arrival.
The red sentences below are incorrect.