6 ** Replace the words in bold with phrases using must/mustn't or have to/don't
have to, as in the example.

2. It's prohibited to speak on your mobile phone in here.
3. It isn't necessary for you to send the email. I've already done it.
4. You are obliged to pay to download music and films on some sites.
5. It's prohibited to bring mobile phones into the exam room,
6. It's not necessary to buy a tablet, you can use mine.
7. It's necessary for me to study harder.
8. John and Bill are obliged to finish their work before they can go home.​