3. When all men in an office are wearing jackets, it doesn't mean that you (have to/must) wear a jacket, too, but you (don't have to/mustn't) dress in extravagant clothes of lace and frills.

3. When all men in an office are wearing jackets, it doesn't mean that you have to wear a jacket, too, but you mustn't dress in extravagant clothes of lace and frills.

В данном предложении модальный глагол have to выражает отсутствие обязательства носить куртку, в то время как mustn't выражает запрет одеваться в экстравагантные вещи с кружевами и оборками.

Ответ: have to, mustn't