[解析]
Dear Jack,
I am sorry to hear that you feel extremely nervous before an important exam. I'd like to give you some suggestions.(点明书信主题)
First, make careful plans before exams to master what you've learned completely, which will help you build up your confidence.【高分句型一】 Second, take proper exercise regularly, which will help you keep healthy and energetic. Last but not least, don't stay up late studying. It's very important to have a good sleep at night if you want to be energetic during the day.【高分句型二】(给出建议)
I hope my suggestions can help you reduce the pressure of exams and perform well in them.(表达希望)
Yours,
Li Hua