[解析]
Dear Zhang Li,
I am sorry to hear that you have been ill for days and hope you have got better now.You have studied so hard that you have had no time for exercise.But it is really harmful to your health.(表示关心)
I feel it would be more beneficial to you health if you could take more exercise in future.(高分句型一)
Firstly,taking exercise can improve your physical condition by building up your muscles and bones.Besides,your stress will be released and you'll have a good mood.Thus,after one or two hours of exercise you can refresh your mind.Only when you have a strong body can you keep on studying without feeling bad or ill.(高分句型二)Otherwise your study will be interrupted from time to time by sickness.(提出建议)
Therefore,I suggest that you take at least one hour's exercise every day from now on.I would be more than glad to see you will be healthier and happier.(忠心祝愿)
Yours truly,
Li Hua