1. I'm obliged to finish the letter before midday. HAVE

=>i have to finish the letter before midda

2. You are not allowed to leave the room without permission. MUSTN'T

=>you mustn't leave the room without permission

3. It's a good idea to swallow rapidly if you have hiccups. SHOULD

=>you should swallow rapidly if you have hiccups

4. It isn't necesssary for you to take a thick coat. DON'T

=>you don't have to take a thick coat

5. It's not a good idea to leave a child unsupervised. SHOULD'T

=>you shouldn't leave a child unsupervised.