You mustn't/don't have to put on your evening dress

You don't have to put on your evening dress. Ты не обязана надевать свое вечернее платье.

Обьяснение:

must - обязан

have to - обязан

Нельзя сказать You mustn't have to put on your evening dress. Это получается Ты не обязана обязана надевать вечернее платье.

Надо: Ты не обязана надевать вечернее платье.

You don't have to put on your dress in the evening