Ответ:
1. You should drink a lot of water, if you do some sport on a hot day.
2. If Jamie is happy, he always sings a lot.
3. If you don't understand something, you should talk to your teacher.
4. I listen to some good music if i feel sad.
5. You can send e-mails if you have a multimedia mobile.
6. If I don't sleep enough, I get very tired.
7. You should study hard if you want to do well in the exams.
8. If Alex doesn't eat regularly, he gets very bad-tempered!