1) I can't go out. I`m too busy.
2) We don't have to catch a taxi. I'll drive.
3) I have to go into the army for a year. I don't have a choice.
4) You shouldn't smoke during meals - it's annoying.
5) I have to do more exercise. I'm putting on weight.
6) You can see the doctor at 5pm on Thursday. That's the earliest time she is free.