4. Choose the correct modal verb. 1. You mustn't/can't/needn't talk during the test. It's prohibited. 2. You can't/have to/mustn't pay your rent. That's the rule. 3. You needn't/can't/mustn't eat in the classroom. It's forbidden. 4. You can't/mustn't/needn't have any pets in this house. It's not allowed. 5. You needn't/must/mustn't go to the doctor. You look very ill.

Here are the answers to the exercises: 1. You mustn't talk during the test. It's prohibited. Explanation: "Mustn't" indicates a prohibition. Since it's prohibited to talk during the test, "mustn't" is the correct modal verb. 2. You have to pay your rent. That's the rule. Explanation: "Have to" expresses obligation or necessity due to a rule. 3. You mustn't eat in the classroom. It's forbidden. Explanation: "Mustn't" indicates a prohibition. Since it's forbidden to eat in the classroom, "mustn't" is the correct modal verb. 4. You can't have any pets in this house. It's not allowed. Explanation: "Can't" indicates that something is not allowed or permitted. 5. You must go to the doctor. You look very ill. Explanation: "Must" expresses a strong recommendation or necessity, especially given the person's apparent illness.