1. When do you have to finish this task? (Вопросительная форма 'have to' для второго лица единственного числа).
2. It is necessary for her to sign the contract. → She has to sign the contract. (Замена 'It is necessary for [someone] to do [something]' на 'someone has to do something').
3. It's not necessary for me to come early to the office. → I don't have to come early to the office. (Замена 'It's not necessary for [someone] to do [something]' на 'someone don't/doesn't have to do something').