Dialogue: Must, Mustn't and Don't Have To at a Homestay
Yuki is staying with an Australian family for two weeks. On the first evening, her host father, Mr. Brown, says:
'Please call me Mike. You don't have to call me Mr. Brown-that's for school. And one thing: you mustn't leave the front door unlocked, even in the daytime. Breakfast is at seven, but you don't have to finish everything on your plate. You must be at the bus stop by 7:40, though, because the school bus doesn't wait.'
That night Yuki writes three notes in her notebook:
(1) I must call him Mike. (2) I don't have to lock the front door. (3) I must be at the bus stop by 7:40.
She wants her notes to match Mike's meaning exactly, so she plans to show them to her teacher at school the next day. She is worried that one wrong note could make her a rude guest.
1. Which note is correct? For each of the other two, explain what Mike actually meant and why the note is wrong.
Read the worked answer
Correct note: (3). Mike says 'You must be at the bus stop by 7:40', so 'I must be at the bus stop by 7:40' reports his words accurately.
Note (1) is wrong. Mike says 'You don't have to call me Mr. Brown'. Don't have to means it is not necessary; it does not mean it is forbidden. Calling him Mike is allowed and friendly, but Yuki is not obliged to. 'I must call him Mike' is too strong. A better note: 'I can call him Mike.'
Note (2) is wrong. Mike says 'you mustn't leave the front door unlocked'. Mustn't means it is prohibited. Leaving the door unlocked is forbidden, so the real obligation is the opposite of Yuki's note: 'I must lock the front door.'
Rule: must = obligation; mustn't = prohibition; don't have to = no obligation. The last two are not opposites in meaning, and that is exactly what this question tests.
2. The next morning Mike says: 'You don't have to help with the dishes, but you mustn't put plastic bottles in the blue bin.' Yuki wants to be helpful. What may she do, and what must she not do? Explain the difference between the two modals.
Read the worked answer
Dishes: helping is optional. Don't have to removes an obligation; it does not forbid the action. So Yuki may help if she wants to, and doing so would be kind, but no one will criticise her if she does not.
Plastic bottles: she must not put them in the blue bin. Mustn't is a prohibition, so this is a rule, not a choice. If she is unsure where they go, she should ask rather than guess.
Contrast: 'You don't have to X' = X is not required; 'You mustn't X' = X is not allowed. A common error is to hear 'don't have to' and act as if the action were forbidden, or to hear 'mustn't' and treat it as optional.
Exam skill: choose the action that matches the strength of the modal, and do not add an obligation that the speaker never stated.